all I had to do was install the new XCode on the Snow Leopard DVD, and then download the iphone SDK 3.0 for Snow Leopard from apple.com/developer (all 404MB of it!).
Do I like the new OS X? sure. I didn't see a lot of difference at first, really. Maybe it seems a little faster, maybe it's placebo. hard to tell. After a few hours I thought to try the new Finder; that is noticeably faster than the one in Leopard. Oh, one bad thing about Snow Leopard: it messed up my customized date and time display in the menu bar clock; I had gone into some plist file years ago to customize it, now I'll have to look that up again.
also, the location feature is neat; you don't have to tell it what time zone you're in any more. also, safari has access to your location too. I've told it to detect my location for this post.
here is the output of sizeof on some standard C types in Mac OS X 10.6.0 (Snow Leopard); woot, 64-bit pointers!
bool: 1 byte ; 8 bits char: 1 byte ; 8 bits unsigned char: 1 byte ; 8 bits short: 2 bytes; 16 bits unsigned short: 2 bytes; 16 bits int: 4 bytes; 32 bits unsigned int: 4 bytes; 32 bits long: 8 bytes; 64 bits unsigned long: 8 bytes; 64 bits long long: 8 bytes; 64 bits unsigned long long: 8 bytes; 64 bits int*: 8 bytes; 64 bits float: 4 bytes; 32 bits double: 8 bytes; 64 bits long double: 16 bytes; 128 bits byte enum: 4 bytes; 32 bits short enum: 4 bytes; 32 bits int enum: 4 bytes; 32 bits