Damn, this is so disappointing. I hope that after the dust settles, he is shown to be innocent of any illegal activity. More details from cyclingnews tidbits:

The Vuelta a España has been rocked this morning by news that Tyler Hamilton has returned two positive blood tests that showed evidence of a homologous blood transfusion. One test was performed at the Athens Olympics and another at the Vuelta on September 13. Both tests showed evidence of a "mixed red blood cell population, an indication of a homologous blood transfusion," Phonak's press officer Georges Lüchinger was quoted by AP as saying. The results of the counter-analyses are not yet known, but are expected today (Tuesday).

Hamilton, winner of the gold medal at the Athens 2004 Olympics in the individual time trial as well as the Vuelta's eighth stage time trial, abandoned the race prior to stage 13 claiming stomach problems. Hamilton has denied having a transfusion, saying the positive test was the result of a surgical intervention he had some time ago. If the B samples are confirmed positive, then he risks losing his Olympic gold medal, which would make Viatcheslav Ekimov the Olympic time trial champion again.

Update: here's video footage of Tyler at a press conference on 9/21. I've read elsewhere that he's already losing sponsorship deals, based on nothing more than what is widely accepted as "suggestive evidence" (ie, not proof).

Another update: here's more information about the test for blood doping, how it works, and how reliable it is (basically, it says that the test is totally accuratem, and therefore implies that Tyler is guilty of blood doping).