In bfgminer, what does Accepted mean? While mining with a pool,
I recently noticed among the numerous "Stratum from pool 0 requested" lines that
bfgminer prints,   one saying 

 Accepted (hex number) CPU 2 Diff 288/2


Does this mean that I found a valid block? Or something else?

Looking at my BTC Guild account, I see no evidence of any blocks found by my computer.
This means your pool server has accepted one block, generated your CPU, third thread, of difficulty 288 (with a minimum of two).