Not that I'm on Snow Leopard yet (ever??), but should you get an error such as:

"hpricot_scan.bundle: mach-o, but wrong architecture"

Or any other gem, doesn't have to be Hpricot, then it is, as the error message says, because of a mismatch between the architecture of Ruby and the gem. And on Snow Leopard this is most likely because you are using a 32-bit version of Ruby and the gem has built as 64-bit (as is the default on Snow Leopard). And this has most likely come about because in 10.5 you followed one of the Hivelogic articles and installed a version of Ruby that is still your default after upgrading to 10.6. The quick way to check is to do

which ruby

if it returns /usr/local/bin/ruby then you need to change your PATH so you are using the Apple installed 64-bit version in /usr/bin, or follow the new Hivelogic article.

If the above isn't your case, then use the file command to find out what the architecture mismatch is:

file /path/to/gem/eg/hpricot_scan.bundle
file /path/to/ruby

32-bit binaries will return Mach-O executable i386 and 64-bit Mach-O 64-bit executable x86_64. Because, as unlikely as it is, it's possible that you are accidentally building your gems as 32 Bit.