My girlfriend has had the HTC Evo Design since about launch time, and just recently her microSD card is ejecting itself randomly. She doesn't notice until she tries to take a picture and it tells her that she needs to insert a microSD card.
My sister has the optimus s, and I've seen that happen with her phone from time to time.
Try backing up the contents then format it. If that doesn't work, try a different sd card and see if the problem persists. If so, then you can rule out a faulty sd card. I haven't experienced this at all on my Design. Did you root it and maybe delete something you shouldn't have?