Harmonix designer Dan Teasdale’s confirmed on his blog that Lego Rock Band was supposed to be announced at GDC. He’s provided the slides to prove it.
There’s a PDF there of an earlier version of the designer’s GDC session – obviously, it didn’t happen. Looks like Lego Rock Band was planned to be announced prior to this year’s show.
Teasdale’s slides show that the game will hit in 2009.