Check out
CNetX Flash Format. It's a pay-for app but you can download the trial which will let you format for a limited time. Worked great for me.
Did you buy the card on eBay? Be warned that nearly all of the flash memory sold on eBay is Chinese pirate cards. They can have problems and even claim (and show) capacities that the memory actually can't support.
I thought had a problem with my Sandisk 4GB microSDHC also. It seemed to work okay in the phone, but I couldn't read or format it in my SD card reader on my laptop using two different adaptors. I was also having trouble copying large files to the card through ActiveSync over USB (the file would constantly keep restarting and never finish copying). I still hadn't gotten to the source of the problem yet. I did find that the card was readable with the same SD card adaptors in another card reader on a different computer so it might be driver related. See if you can find updated drivers for your reader. I'm not sure what the ActiveSync problem is.