Okay, I've searched for hours and hours and have attemped to solve this on my own for an eternity, but I fail. I even tried to upgrade to the newest firmware, but instead got me a 507 error (major headache) which took about half of my life to fix. So now I turn to you guys for help. I just got a BB 8100 T-Mobile last week, and bought a 2GB Crucial MicroSD to use with it.
These are the symptoms and should answer most of your questions..

1. There is no 'Format Card' option, in fact, it says 'A media card is not presently inserted in the device', which, I'm guessing, would hinder the 'Format Card' option.
2. These are my settings under the Options -> Advanced Options -> Media Card subfolder. Media Card Support - On, Encryption Mode - None, MSMS - On, Auto Enable - No.
3. This is the firmware -- v.4.2.1.107 (platform 2.3.0.83)
4. Yes, I did format this card on the computer, no luck.
5. The card is not broken, computer can read the SD adapter and I can transfer things onto the card via the computer.

Thats pretty much all I can think of right now. I don't want to go out and buy another card if this card works fine.. So hopefully one of you guys will be able to help me out. Thanks!

With everything stated it sounds more like a BlackBerry Device issue. Can you run it to a T-Mobile store, try your card in a different 8100?

I was planning on using the card into a buddy's BB Curve to see if his would work and vice versa. I'll let you know how it goes tomorrow.

I also posted up this problem here: Micro SD Problem - PinStack.com - BlackBerry forums
Maybe it will shed some more light on my problem? I don't know. Help!

Great thanks

I had the same problem. I took out the battery out for a few seconds and let phone boot up. My issues went away. Hope this helps. As I felt your frustration.

You mean by the battery pull right? Where you take it out for like 10 seconds, pop it back in, and turn it back on?
I've tried that numerous times with no changes whatsoever. Thanks for your help though.

I had the same problem tryed 3 different make of cards then put in a 2Gb sandisk problem sorted worked perfect first time.Just abit funny that 3 others told me to format & nothing happened or did'nt even show.

Oh yeah my card reader picked up everyone aswell just the phone would'nt.

UPDATE

Today I let my friend use my Crucial card while he let me use his sandisk card. His BB Curve read both cards fine, while mine read neither.
I took my phone to the store today and they said that they would give me a new phone. Hopefully this will be resolved by then. But I'm just curious as to what the problem is even though I'm getting a new phone. My first concern is that the new phone will be just like this one and not be able to read the card. What do I do then?

While standing there with the new phone in hand at the store, insert the card and see if it will work.

And if it doesn't work and they try to tell you it's your card, make them use one of theirs (even a brand new one - I've done that to em before) and test it... Or take your friend/friend's card in with you as well... Be fully prepared!

Thanks for the help guys. I'll be sure to update you as soon as I get the new phone.