Harmonix seems to have been planning to announce the oft-rumored LEGO Rock Band at GDC, according to an early draft of a slideshow presented by Harmonix senior designer Dan Teasdale.
As far as rumored LEGO games go, LEGO Harry Potter has to exist. It'd make too much sense (and money) not to. LEGO Rock Band, though? That's always seemed a bit far fetched. What would Rock Band gain from just making everything LEGO-ized? Still, the rumors have never really gone away, and they may have some genuine credence: by the looks of things, Harmonix were planning on announcing it at GDC.
The confirmation comes from an early draft of a slideshow presentation (posted online, now no longer so) given at GDC by Harmonix senior designer Dan Teasdale. One of those slides shows a timeline from 2006 to 2009 of music games released and due for release. Sandwiched between the words PSP (which would refer to Rock Band Unplugged) and Beatles (Beatles: Rock Band) on the 2009 end of the timeline is the word "Lego."
Furthermore, a later slide includes a placeholder image for a project "to be announced before or at 9 AM Friday," as well as a note in which Teasdale refers to an unannounced title that he will be showing at GDC. Seems like pretty darn conclusive evidence to me, unless it's all some elaborate ruse designed to make us all look like idiots.
Teasdale's slideshow also makes mention of a number of unannounced Guitar Hero titles, specifically referring to a Guitar Hero: VH and a Guitar Hero: BH. How exactly Teasdale knows what Activision is up to aside, VH would probably mean Guitar Hero: Van Halen, but BH? I'm not really sure about that one. Bass Hero? Baha Men?