I meant that if, for example, you agreed with a stranger to buy x BTC for y USD over, say, paypal, then they could pay you, then you could not give them the BTC, or you could give them the BTC and they could complain to paypal. Paypal would be likely to reverse the transaction, and you would find it very difficult to prove that you were in the right. You need trust in the other party for a transaction to be safe, and big exchanges like MtGox are inherently more trustworthy than an anonymous person you meet in an IRC, for example.

Hmm, yeah I wasn't too sure how mining worked, I guess I still need to learn more about that side of bitcoins.