You can definitely do it. But you either can't use your home folder to store your data for the most part, or you have to figure out how to do some kind of voodoo.
What I did is not supported by Apple, but it works just fine. And you can take it in for repair under warranty no problem. So there's nothing to stop you from doing it.
I got the information I needed here:
Basically the idea is you copy your user folder to a big conventional drive. Then you install Lion on a new SSD and create a user account with the same name. I named the big drive "Users" and then put the Home folders in its root.
Go to Users& Groups in System Preferences. Right click on the account. Choose advanced options. For the Home directory field, there's a "Choose" button on the right. Click it and navigate to the copy of the account on the new User drive, and select it. Reboot. You will now be using the user account on the conventional drive. Your Desktop, Pictures, Music, Downloads, Documents, etc. folders are now on the big drive, but it's just like it's on the boot drive.
A big benefit here is that you can format your boot drive and reconnect it to the User drive, and not touch your files at all. And you can clone the User drive and it won't contain your system files. (Or your Applications which remain on the boot drive.)
If it's not clear, ask questions and I'll try to clarify.