No store update yesterday and no comment from SCEE regarding this. As such GT5: Prologue is not available for download yet.
There may be a number of reasons for this and the rumour doing the mill, is the recent unauthorised access to some customers PSN account.
Link
No official word has been given yet by SCEE, but
Eurogamer are trying to get Sony to explain why this hasn't been made available for download.
Menawhile the official UK Playstation 3 pages suggest that it has been released.
Link
However, people still don't see any available store updates as of yet.
As soon as there are any further developments and/or updates, I will post and let you know.
- Edit-
As I speak the store is being updated now. Some people will get disconnected and won't be able to see the new content straight away and some people just logging can see the new content.