I'm a newbie to blackberry and have only had my 8130 for a couple of weeks now (through Alltel). I also have a SanDisk 2GB MicroSD card that I purchased for it.
As I read e-mails, or just move from screen to screen on the device I regularly get the "Media card removed" immediately followed by the "Media card inserted" message. I have removed and check the card several times but it continues to do this. I have also upgraded to the latest OS in hopes that would resolve the issue, but it still happens.
Try placing a thin business card (or tear one in half) on top of the media card silver gate, replace the battery and see if that might hold it in tighter.
The card on the 8130 enters through the side and clicks into place. It goes in pretty hard (it feels like a very tight fit), and doesn't move when I try to move it with my finger. The cover also closes properly once the card has been inserted.
I have tried removing and replacing the battery while the card is in, after, etc. with no change.
Be sure when you insert the card to push all the way in. The card sets against a spring and has to be depressed again to remove, so be sure it's locked in place. You have to use a fingernail to insert it all the way.