It is a timing problem on the interface level
when I bought my fist 3G, I've tried it with a 8GB µSD Card and I've seen the symptoms described by other user (freeze problem, very very slow motion, ...). Without µSD card, it was working perfectly.
I was sure this µSD card was working fine in a HTC HD, so I tried a slower µSD card and it was working again. But because I was unhappy with the fact that I had to slow down the 3G (by using a slower card) to get it working, I've tried to find other 3G to try my SD card... and finally, I tried it in 11 HTC 3G (that was "working" according to their owner) and on 3 of them, the SD card did not work!
So, I've exchanged my 3G and with the new one, my SD card is working perfectly.
During the electronic design of the HTC 3G, the µSD interface timings were borderline. So, if you are lucky, your model supports fast µSD card, if you are not, you have to buy a slower µSD card or try to exchange your HTC 3G. Anyway, it is not a "critical" problem because it has a simple solution... But for sure, everybody prefers to be lucky