EA has declined to comment on what may be accidental confirmation of LEGO Rock Band.
The evidence comes from an early set of GDC presentation slides sent to Activision for approval by Harmonix designer Dan Teasedale (and spotted by Kotaku). One of these, titled 'External Landscape', depicts a time-line of Rock Band and Guitar Hero products. And there, lo-and-behold - in-between the (presumably fiscal) years 2008 and 2009, and Rock Band PSP and Beatles games - is a block that reads "Lego".
A later slide then features a placeholder image for a "project to be announced before 9am Friday". This, presumably, was to be LEGO Rock Band.
But, of course, none of this was ever announced at GDC.
Also worth noting are the unannounced Guitar Hero games included on the slide. There's no way of knowing how much Teasedale is privy to, but GH: VH could well be Guitar Hero: Van Halen, while GH: BH may turn out to be Guitar Hero: Band Hero. These, alongside Guitar Hero: Arcade, are due out later this year according to the slide.