Hi Chris,
When you call HTTP::payload from the HTTP_REQUEST event, only the portion of the HTTP payload in the first TCP packet is available, but the full payload (which could be in multiple packets) is not available. If you want to do pool selection based on HTTP payload content, you'd need to use HTTP::collect to buffer the full payload and do the inspection/pool selection in the HTTP_REQUEST_DATA event.
For details you can check the HTTP::collect wiki page:
http://devcentral.f5.com/wiki/default.aspx/iRules/http__collect
Note that the pool command isn't listed as a valid command in the HTTP_REQUEST_DATA event, but I think this is an omission on the wiki page--not an actual limitation.
Aaron