Instead of mousing to the ‘Check For Updates’ text button at the bottom of the Apps section of iTunes 10, you can simply hit ⌘-R to run the check.

Strangely, I cannot find this command anywhere iTunes’ menu system besides the main app menu, but that’s for checking on iTunes updates, not app updates. Plus, ⌘-R is set for the Store > Reload Page command, but then, only when you’re in the store; it’s grayed out when viewing your apps.