My wife and I just got MyTouch 3G phones 4 days ago and are BOTH getting replacements for this same reason. My SD card has actually gotten worse. It no longer is "fixed" with a power cycle - it just says "Memory card blank" and won't mount or format or anything, completely useless. I went with having TM send new phones for the simple reason that there is a 14 day buyers remorse period that resets every time you get a new phone. So when the new ones come I'll have another 14 days to be able to get fully refunded or a replacement (with another 14 days). If I had HTC send cards out the 14 days would probably expire before I even got the new cards.
Also, TM did admit this is a known issue, but only for "the last few days" !!! They are selling phones with defective parts and they know it!
At any rate, I really like the MT3G when its working!