I'm going to go out on a limb here and offer an opinion first. While you will likely get multiple elegant methods from here to achieve this goal, I'd like to first ask what your reason is? I know a few Windows users do this to prevent any OS corruption on the C: drive not affect their data on the D: drive, and allow them to reformat the C: drive periodically to reinstall Windows. While this may or may not be necessary in Windows is up for discussion, but this practice is certainly not very common in OS X, and I find that splitting up your main OS X installation into two partitions very restrictive.
What I would suggest is keeping a regular backup of at least the
/Users folder. That way, if you do need to reinstall OS X, you can migrate your
/Users folder back on to the fresh installation, either manually or using the Migration Assistant.
Having said that, if you would like to keep your data on the other partition, I would keep the
/Users folder on the main partition for sure, and even keep the canonical folders in there (i.e. Documents, Pictures, Library, Downloads, Movies, Music, Public, and Sites). Then, within these folders, you can move specific things to your other partition.
For example, libraries used by Apple programs can be individually moved to a folder on your other partition following instructions on the Internet (e.g. iTunes, iPhoto, Aperture). You can create another Downloads folder and then set the download preference in each browser you use. And then, of course, you can store documents that are not directly managed by an application on the other partition without any trouble.
The key thing to understand is that any application that has a default location for managed files (e.g. Journler, VMware Fusion) will need to be dealt with individually to move your data onto your other partition.
To present another option, I'm thinking someone with more of a UNIX or Linux background will be able to suggest a method of modifying
/etc/fstab in OS X 10.6 to make it such that the entire
/Users folder is mounted from the other partition.
3. Much Better Solution
As cOle2's link suggests, you can move a specific user's folder to another location by using the Accounts Preference pane.