Generally speaking, my recommendation is to delete photos from your digital camera manually after you're certain they've been imported into iPhoto. That's easy on every camera I've used, so the peace of mind is worth the small extra step. However, since most photos I take using my iPhone aren't that impressive, I've always let iPhoto delete the photos automatically after import, with no problems.
However, after importing 162 photos and movies from my iPhone today, I accidentally clicked the Keep Photos button in iPhoto, leaving all the already-imported photos on my iPhone. iPhoto would happily hide them from view when importing new photos, but it wouldn't delete them. Needless to say, I didn't want to delete 162 photos one at a time by tapping the trash icon followed by the Delete Photo button - I have better things to do than tap my iPhone 324 times (like write this article!).
I figured out two ways around the problem, one using just the iPhone and another using Image Capture on the Mac.
Once again, it's a little surprising that there's a capability of the age-old Image Capture program that Apple hasn't yet built into iPhoto, but there it is.