As you can tell by the fact you still can't buy it, WipEout HD has been delayed. Rumours suggest the reason for the hold-up could be epilepsy tests, it could be glitchy online play, it could be anything and everything in between. But what do Sony have to say about the whole thing? They've got a very sensible answer:
The reason for the delay of WipEout HD on PS3 has been due to the many improvements we have been making and the numerous features we've added to the game. These include: 8 reverse tracks, 4 extra ships, 2 alternative HUDS, 2 Player offline split screen and XMB Trophy support. We look forward to sharing WipEout HD with you very soon. Regarding the speculation about health issues, we can assure you that we take consumer safety very seriously and monitor it very carefully.
That last line doesn't entirely rule out the epilepsy stuff, but that first paragraph is making the wait sound worth it regardless. Wipeout HD Delay Response Confirms Trophies & Splitscreen News [TVG]