Well, looking back on recent history with the past several games' Japanese release dates:
Ocarina of Time: 21 November 1998
Majora's Mask: 27 April 2000
Wind Waker: 13 December 2002
Four Swords Adventures: 18 March 2004
Twilight Princess: 2 December 2006 (US: 19 November)
Oracle of Ages &
Seasons: 27 February 2001
Four Swords: 14 March 2003
Minish Cap: 4 November 2004
Phantom Hourglass: 23 June 2007
There's more or less a basic pattern throughout all of these releases. More or less, for each platform -- console or handheld -- there's around a two-year development cycle (± ½-1 year). Now,
Twilight has a really weird history because it was announced
well in advance... (2004, was it?), but it kept facing delays that kept pushing it further and further back in the dev cycle... making its release a full 2-3/4 years after
Adventures, which -- while a fun game -- isn't exactly your standard modern-day console Zelda game.
Judging by those figures, the earliest you'd expect a new console Zelda to be released would be December 2008, but considering there's nothing on the radar, I'd say you'd expect another release by mid 2009. That means, of course, that we should expect to hear announcements for it perhaps starting the middle of this year.
Just a wild hunch, of course. Really hard to say one way or the other at this point.