I gathered data on transaction first-confirmation times this weekend:
https://docs.google.com/open?id=0B74FtkFP4j73TXZUY05kSFVzN00
Each line in the file (except the first one) represents one transaction. Each column is:
Number of seconds the transaction spent in the memory pool.
Size, in bytes, of the transaction.
Fees attached to the transaction (in satoshis).
Priority (floating-point number) of the transaction when it first entered the memory pool. Priority 0.0 transactions have inputs that aren't yet confirmed.
Last column is "1" if the transaction exited the memory pool normally, by being included in the a block, or "0" if the transaction was still in the memory pool when I shut down bitcoind.
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I haven't really analyzed it yet (feel free to help if you're a data geek); I plan on testing some algorithms for suggesting a reasonable fee if you want your transaction to get confirmed quickly, and suggesting an estimate of how long you'll have to wait if you don't attach any fee at all.
But it look like 35% of transactions this past weekend got into a block within 10 minutes, and 87% got into a block within one hour.