Tim007 tells us this in the Movies Spoofs Forum:
SAT.1, a German TV-Station, has just reported, that the producers of the 3. Austin-Powers movie asked Pierce Brosnan to play a guest role in Austin Powers 3 – as James Bond!
Further SAT.1 reported, that Pierce has spoken with the Bond production firms – and they said “yes”. So Pierce Brosnan will play James Bond in Austin Powers 3!
DarkHorizon's have also reported the news saying:
Austin Powers: Goldmember: WENN reports that while Tom Cruise may be out, Pierce Brosnan looks to be in to cameo in the upcoming third AP comedy. Brosnan “will spoof his on-screen image in the sequel playing play a promiscuous secret agent who puts Austin’s bedroom exploits to shame”. A worker on the film says “It’ll be obvious that Pierce is playing James Bond – but Bond executives will ultimately decide whether Bond’s name or 007 will be mentioned on-screen”.
Despite this news it's common knowledge that Brosnan was to make a similair appearance in Austin Powers: The Spy Who Shagged Me. However, due to time contraints he couldn't film the required scenes. Austin Powers: Goldmember begins filming in January, at the same time as Bond 20, so it's very possible that time constraints may alter Mike Myers' hopes again.