Given that I've been using a desktop and a Mac notebook since 1992, I have to respectfully disagree with bdkjones because for some people two computers really is necessary. Yes, there's software that will let you keep the two computers synced but there are a few caveats.
1) don't try to sync mail. Use IMAP which keeps your mail on the server instead of downloading it to your computer.
2) don't try to sync iTunes or iPhoto. As much as possible, make one of your computers the base for your music and photos. Copy a subset of your music and photo files to the other computer and import them into iTunes and iPhoto there.
3) don't try to sync the OS or software. Perform individual installations of each
4) take a look at ChronoSync and give it a test drive. There are other programs and I've tried many over the years, this is the one I like the best. Of course, YMMV so hit macupdate.com and search on
sync to find lots of other programs. Test them too. Nearly all of them offer full functioning time limited trials. Use this software to keep specific folders in your Home folder synced but don't attempt syncing your entire home. Leave out Music, Movies, for example Library. Also, you might want to sync your Pictures folder but leave out the iPhoto library.
5) .Mac has one feature that keeps me paying my fee each year - sync services. This will keep your bookmarks, address book, and iCal synced. MarkSpace also makes a program that does this but I wasn't impressed with version 1.
Owning two computers does require a bit of discipline. If you have a proposal you wrote on the MacBook you don't want to start some editing on the iMac and do some more editing on the MacBook the next day if you haven't synced. This is the route to madness. For me, the MacBook has one set of specific tasks and my desktop another. I create copy on the MacBook and rarely write on my desktop. I do photo editing and graphic work on my desktop and rarely on my notebook. The only work I do with InDesign on my MacBook is last minute editing. This discipline keeps me from duplicating work.
BTW, my wife has recently developed a different solution. She has an 8 GB flash disk and she saves her files to it. At the end of the day at work she syncs the flash drive to her hard drive of her desktop and at home does the same thing with her notebook. So she actually uses the flash drive as her drive and her computer hard drives as backups.