Not hard to do. You'll just want to copy your Entourage database to the external firewire drive and put an alias to the external copy back in the original location. I'll assume Entourage v.X under OS X, since you stated neither:

Quit Entourage X.
Open the Process Viewer (/Applications/Utilities) and Quit the Microsoft Database Daemon process if it is running.
Look in your Home Documents/Microsoft User Data/Office X Identities folder and locate the "Main Identity" folder (should be only folder there unless you are using multiple identities.)
Copy that "Main Identity" folder to your external firewire drive, and name the copied folder something else; e.g. "Portable Mail" or whatever you like.
Make an alias for that "Portable Mail" folder, and place the Alias back in the Documents/Microsoft User Data/Office X Identities folder where your "Main identity" folder is.

Launch Entourage.
Select "Switch Identity" from the Entourage menu. The Identity list should have "Main Identity" and "Portable Mail". Pick Portable Mail.

You should now be using the database on the external firewire drive.
Rename the original "Main Identity" folder to "Local Mail Backup". You shouldn't be needing it, but hang on to it for a few days just in case.

Rememeber to eject the drive or shut down before unplugging it. To eject the drive you may need to Quit not only Entourage but also the Microsoft Database Daemon again to eject the drive without getting an "the drive is in use" message. (I have a simple AppleScript that can Quit Entourage and the Database Daemon and eject the disk that you could drop in the Dock for quick access. I can give it to you if you need it.) Obviously you don't need it if you shutdown before removing the external firewire drive.

When you connect the drive to your work machine:
Make an alias to the "Portable Mail" folder on the external drive and place it in that machine's "Office X Identities" folder (home Documents/Microsoft User Data), too. Use Switch Identity... in Entourage to switch to "Portable Mail" on this machine, too. Then rename that machine's old "Main Identity" folder, too.

That should cover it.

Note with either machine if you log in without the firewire drive connected your Entourgae won't work properly (it might re-initialize itself with a new, empty "Main Identity", in which case you'll need to "Switch Identity" again to go back to the firewire drive's "Portable Mail" database. And of course you won't see your mail without the firewire connected, and you won't get appointment reminders (since the calendar's in the same database.)

As long as your wife has her own eMail account, she may well be better off using her own Entourage identity on the home system. That will keep all her data separate from yours. The two Entourage databases (which include mail, address book, calendar, tasks, notes, rules, signatures, etc.) would be hard to keep in sync if you're both reading mail from the same account or trying to keep identical address books/calendars. (Since she'll start with a copy of the current database, it will start identical, but soon get out of sync as they both get modified separately.) You should delete your eMail account from her identity database configuration so she doesn't accidentally (or automatically from a Schedule) pull your mail into her identity, and you should likewise remove her eMail account from the Portable Mail identity. (This is done by selecting Accounts from the Tools menu.)

(BTW, I still suggest renaming her "Main Identity" folder to her name or something else she likes. Why? Since you'll be keeping your data on the removable drive, it's not inconceivable Entourage may be started without the proper database attached. Sometimes when Entourage gets confused about finding its database, it reflexively (trying to be helpful) recreates a new, empty one called "Main Identity". Renaming her active database makes sure that reflex doesn't tragically re-initialize your wife's data.)

(Synchronization hint: If you do what to share a few contacts or move a couple of eMail items between the identities, you can drag a message or contact from its respective list pane in Entourage and drop it into Finder (say, onto the desktop, or into a Finder folder.) That creates a .eml (mail) or .vcf (contact) file for each item. Once you switch to the other identity you can drag the file back into Entourage and it will get added to the current identity's database.)

By the way, while Entourage keeps almost everything (even most of its preferences) in the identity database, there are a few things kept in your account library: mail account passwords are stored in your Keychain, so when you use the Portable Mail database on the second mac you'll probably have to re-enter the passwords on that mac. It also keeps cookie settings and cookie-related preferences in the library (com.apple.internetconfig.plist), which it shares with Internet Explorer; you're not likely to notice that unless you use different "accept cookie" rules on the two systems or use Hotmail acocunts.

If your system at work is on a company LAN, they may have firewall filters in place to prevent you (or any other employee) from contacting the SMTP server of your ISP to send mail. You'd need to switch the SMTP server in your mail account (Tools>Accounts) to the company's SMTP server. You'd need to ask the company IT people what that address is. (It's also not impossible they'd have a block on the POP server ports, too; depands on the company's IT policies, etc.

I am currently using this method with entourage.. but want to switch back to apple mail using the same system and can't get it to function... is it possible with Leopard to do this or is there another way??