MtGox withdrawal transaction not in blockchain
Unfortunately Mt.gox has tendency to delay BTC withdrawals from time to time. Happened to me couple of days ago too. I tried to withdraw 1.8 BTC to another address but the transaction didn't show up in the block chain. Eventually it did, something like 12 hours later. I heard other people have waited for days for their withdrawal to complete.

I recommend to open a ticket just in case, but realize that this is not uncommon.
Hope this helps ;)