What sorts of problems can be caused by a compromised private RSA key? Is this even that big of a deal? I'm getting very little specific info on Google. :(
Well, it depends on what your RSA key is used for, but yes, this is kind of a big deal.
Any data encrypted using the public key can be decrypted by using the private key.
If the key was used for authentication, then anyone with it can impersonate your identity on sites that were using that key.
If your key was compromised, you should generate a new key and revoke the old one before anyone can impersonate you or decrypt the protected data.