"Back-grading" from Lion to Snow Leopard

No. Same as with any prior version of OS X. You cannot downgrade without starting from scratch. On the plus side this is your best excuse to finally make a backup.

Downgrade Lion to Snow Leopard

1. Boot from your Snow Leopard Installer Disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu.

2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area. If it does not say "Verified" then the drive is failing or has failed and will need replacing. SMART info will not be reported on external drives. Otherwise, click on the Partition tab in the DU main window.

3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.

4. Quit DU and return to the installer. Install Snow Leopard.

This will erase the whole drive so be sure to backup your files if you don't have a backup already. If you have performed a TM backup using Lion be aware that you cannot restore from that backup in Snow Leopard (see below.) I suggest you make a separate backup using Carbon Copy Cloner 3.4.1.

If you have Snow Leopard Time Machine backups, do a full system restore per #14 in Time Machine - Frequently Asked Questions. If you have subsequent backups from Lion, you can restore newer items selectively, via the "Star Wars" display, per #15 there, but be careful; some Snow Leopard apps may not work with the Lion files.

Jeff,

There are some details you need to do first in 10.7 before you downgrade so your files are free and work in 10.6.

I have a more detailed method here to make sure your 10.7 to 10.6 transition is more smoother process.

One big tip is you can't use a 10.7 TimeMachine to restore a User to 10.6 with, as 10.7 alteres the User/Library folder containing your settings and stuff.

Someone here has tried that and failed, why I wrote this more detailed method to prevent this and other potential issues, like making sure the ENTIRE drive is erased before installing 10.6 again. Plus some performance tips about hard drives and the install order

How to revert your Mac to Snow Leopard

You might have noticed that's already covered in my prior post:

"This will erase the whole drive so be sure to backup your files if you don't have a backup already. If you have performed a TM backup using Lion be aware that you cannot restore from that backup in Snow Leopard (see below.) I suggest you make a separate backup using Carbon Copy Cloner 3.4.1.

If you have Snow Leopard Time Machine backups, do a full system restore per #14 in Time Machine - Frequently Asked Questions. If you have subsequent backups from Lion, you can restore newer items selectively, via the "Star Wars" display, per #15 there, but be careful; some Snow Leopard apps may not work with the Lion files."

but be careful; some Snow Leopard apps may not work with the Lion files.

I covered this a little bit better in my UT and included a process that doesn't include TM which not everyone has.