My daughter's phone had the same issue last week. She never removes the SD card. I unmounted and remounted and her missing apps came back.
I've had the exact same issue with my Vortex and I actually took one back thinking it was a lemon. But alas, it's no lemon...it's probably a software problem, but after I rooted my phone (gingerbreak) the problem went away. Verizon tech support has no solution and neither do the idiots at the stores.