/cast Dire Bear Form (or whatever your target form is)
Fun, eh? This works because /cancelform is going to be recognized instantly in 2.3, like /dismount is now. The macro should function no matter what form you're currently in.
This also allows things like shifting from bear to caster, drinking a pot and/or using a healthstone, and shifting back into bear, all without interruptions and with only one keypress:
/use Super Healing Potion
/use Master Healthstone
/cast Dire Bear Form
The 2.2 patch notes included an item that seems related to this ("Cast sequence macros no longer get stuck at feral druid abilities"), but I think that's just a bug fix; the above macros don't work in 2.2 as far as I know. Here's my question, though: if you can make a macro to shift forms directly, why not just make the default form buttons have that same behavior? Maybe they will, but I haven't heard anything about it yet.
[macros by Nandini of Darrowmere]