I believe the Broad Institute at MIT is doing metagenomics on this very question. Obviously, they will sequence as much as they can.  Unfortunately, you can't go directly from one of those sequences to a Petri dish with a culturable organism.  For the ones that grow, I guess you could do colony lifts and use an oligonucleotide probe.

If you are looking to get a measure of diversity of gut biota you might try pyrosequencing.