The process was pretty straightforward, though I went through it twice. I thought a quick write up might be helpful for someone else since there were a couple points that gave me pause.

I bought the Snow Leopard CD from Apple for $29. I decided to reformat my drive entirely rather than simply upgrade -- my mac is coming on three years old, why keep around all that cruft that's gathered over the years.

A few posts I read said you needed restart the computer while holding down "c" with the CD installed in order to reformat: I did not need to do this the first time (when I was still on Leopard). I did need to do this the second time, when I had already upgraded to Snow Leopard and wanted to repeat the process. I was able to insert the CD, double click the "Install" option, and then choose "Disk Utility" from the utilities menu at the top of the screen. I spent a few minutes looking for "Utilities" on the install dialog box and didn't see it. It was perhaps too well designed: subtle enough to be easily missed.

I selected my hard drive and chose "erase", then selected "Zero Pass" erase from the "security options" dialog. I did not feel the need for a 7-pass government-certified erasure: it's only my cruft I'm getting rid of, not national secrets. This took a few hours, so I did it before bed.

Once that was done, I quit disk utility and clicked the "install now" icon on the Snow Leopard modal dialog box. Install took a while (hour or so) and then started up smoothly.

I immediately ran "Software Updates", which brought me up to 10.6.3, and allowed my magic mouse to work.

Regarding Time Machine. I was a little uncertain at this step and tried once to migrate my user account from my old machine using Migration Assistant. I had to rename the account since my new account had the same name. I was able to select which files to bring over, though for some, like Applications, it was all or nothing.

When this was complete I was disappointed: each of the copied over directories had a little red circle and x through it: the message I got was that I did not have permission to access these files. Was it because my previous account was an admin and I was an admin as well? Would I have to log out and log back in as the old user (now renamed to sarahleopard)? I didn't like it. Plus, it had brought over all my apps (at my own request) and I didn't want a bunch of them. I also read that if you do this, you risk over-riding newer versions of the apps with your backups. So I decided to wipe and reformat again -- and get it right this time.

The second time, after following the exact same steps up until the Time Machine part, I simply accessed my Time Machine not through Migration Assistant but just manually by clicking through to the directories I wanted from the most recent backup: Backups.backupdb -> Sarah Gray... MacBook Pro ... 2010-04025-151235... Macintosh HD... Users... sarah... Documents... And then dragged over the Documents I wanted to my hard drive. I did the same for Applications, only bringing over my copy of Office.

My final question was, would using the same external backup drive for TimeMachine on Snow Leopard that I had used on Leopard overwrite my previous Leopard backups? I had seen a couple of scary threads from users who believed they had lost all their data doing this. So just to be safe, I called Apple Support and confirmed directly that you can use the same hard drive and not lose your previous backups (i.e., not have the hard drive reformat to the new OS).

I tried it, backed-up my new, spartan and clean Snow Leopard setup and can still easily access my previous Leopard states.

All good. I hope this helps someone embarking on the same task.