Currently available version of Google Talk Video plug-in for Mac OS X (188.8.131.524) might cause up to a 30-second delay while putting the computer to sleep under Snow Leopard.
This has been reported and discussed in the following Apple support thread. In the end, the solution typically requires to uninstall Google Talk Video. The solution was found and reported by Snoop Dogg in that same thread.
Basically, the misbehaving process can be found by running
sudo pmset -g log then looking for the process with the highest response time. In my case:
$ sudo pmset -g log ... * Domain: applicationresponse.timedout - Message: Kernel GoogleTalkPlugin com.apple.powermanagement.applicationrespons e.timedout 30000 ms - Time: 9/2/09 12:42:47 AM GMT+02:00 - Signature: GoogleTalkPlugin - UUID: AD1E9199-B66D-41CB-BF4F-590EF232DE79 - Result: Noop - Response time (ms): 30000 ...