The Google Mobile Blog has a tip for Blackberry 8800, Samsung Blackjack, and Nokia N95 owners…
Fast tip: one-click access to Google mobile applications on your phone
My Dash (Windows Mobile 6 Standard Edition) does not have the option to assign buttons or other physical inputs (volume slider) to applications. However, all Professional Edition (Pocket PC Phone Edition) devices do have this feature. You can find it in Settings->Buttons.