If you have any data on the card you don't want to loose try this BEFORE you try formating the card.
1st try another card... beg/barrow another mini sd from someone or just buy a new one... what's a 2 gig go for these day's... bout 20 bucks?
If a another KNOW GOOD card wont read then re-flash the ROM... this probably won't help but it won't hurt to try either.
If the above fails then its a bad card reader... I went through this with my 6700 just over a month ago and sprint had to replace mine... on the up side it cost me nothing(I have insurance) and the referb I received was FLAWLESS... perfect screen, not one scratch on it or the case... I can't say the same for my old phone
If you don't have insurance then try the yellow pages for cell phone repair... it may be something simple like a contact the needs tweaking/cleaning.
Good luck,
John