Noone wants to get their feet dirty and kick this forum off eh? Well I'm the man for the job!
Anyways, as the title suggests, here is my prediction for the release dates of the next set of consoles.
North American Console Release Date Scheme
N64-Sep. 1996 Playstation-Sep. 1995
~5 years~ ~5 years~
Gamecube-Nov. 2001 PS2-Oct. 2000 XBOX-Nov. 2001
~5 years~ ~6 years~ ~4 years~
Wii-Nov. 2006 PS3-Nov. 2006 XBOX360-Nov. 2005
~5 years~ ~5 years~ ~????~
????-Holidays 2011 ????-Holidays 2011 Depends if Natal is a success, varies
My personal prediction is that we will see the next Nintendo released in holidays 2012 at the latest, and holidays 2011 at the earliest. Sony will probably want to give Nintendo competition ASAP when the floodgates open, although there isn't much of a trend to follow for Sony or Microsoft, it's reasonably logical to assume that that's the case. As for Microsoft, they have said project Natal is their next-gen. My guess is that if Natal fails, they will pump out a new system and try to make it sound like they didn't say that Natal is next-gen, and if not, they'll milk it for awhile.
A 2012 release for the next Nintendo system would probably be worst-case scenario, and if Wii sales are still super high they might make it backwards compatible, assuming it uses motion-sensing.
Edit: I don't think they will unveil new home consoles at this year's E3, but Nintendo might show the DS's next-gen successor, and Sony might show a PSP 2 or something. Microsoft might try competing in the handheld market, who knows.