This is very impressing. I was able to cop files from my laptop (vista) through a USB cable to my 8 GB card at 3.7 MB/s. this 3 times faster than what I have experienced with my Curve.
I remember hearing somewhere before the Bold's release that it was going to have a "high speed" USB 2.0 MiniUSB port, and that this was a big deal as the previous incarnations had all been USB 1.0/1.1.
I'd guess this is why you're seeing the impressive transfer rates.