It's not your imagination.
For Heap, it would be a bit unwise to ignore IE < 9 still since they're limiting themselves from the start.
A lot of new projects on the front page lately don't work on IE9 either. I lost count of the number of shiny new things (usually involving HTML5) that just show a plain black (/white/gray/blue etc...) screen on IE9 and I'd need to fire up Firefox or Chrome just to look at it.
To be fair, not all of these are catering to IE based browsers (and I dread to think how some of these would look on a Nokia mobile... or, heaven forbid, a BlackBerry), but they're effectively ignoring a very, very, very large percentage of users worldwide -- of not just IE -- to be first out of the gate. BlackBerry in particular is still very popular in Latin America and Southeast Asia.
That foot in the door is all important, I suppose, since the buzz will help sustain at least an initial signup for whatever it is.
The web applications in particular are exactly that. Because they're applications they're crafted to a very select set of platforms since they're executed. For better or for worse, a lot of new projects are already going in the direction of the browser as the OS.
I presume backwards compatibility will be tacked on later and maybe removed as those browsers fall further behind.