Here. The game's now up on the European PSN and is available for download.
GT5: Prologue was supposed to release yesterday on PS3's online service, but was delayed due to "technical" problems, according to a statement issued by SCEE earlier today.
The wait's over, anyway. Go fetch, assuming you haven't bought the disc out of wild frustration at not being able to download it at midnight last night.
Update: SCEE just made the following statement:
We would like to apologise for the lack of a PS Store refresh yesterday. As previously mentioned, we unfortunately experienced some last minute technical difficulties and were unable to publish the new content as planned. However, we have been working hard to resolve these issues and are pleased to be able to let you know that the... content is now live on the UK store.