Hello, i have the same problem from yesterday, after installing some aplications on my phone. It works fine for 1 week and from yesterday I need to pull out the card and put back to recognize it. I got restore the phone to factory settings, format the sd card, the same problem, update the software to 2.3.5 and the same.
If it was the sd card I should have the problem from the first time, not?
I was loking for this problem on forums, but I got no solution.
Problem solved, a new 16gb card and everything is ok!