There are also some odd rules mentioned in the Wiki that increase fees when the queue of transactions gets too deep. I haven't reviewed the Bitcoin code to comment further how these are enforced by miners and when sending in the client:
"If the blocksize (size of all transactions currently waiting to be included in a block) is less than 27 kB, transactions are free.
If the blocksize is more than 250 kB, transactions get increasingly more expensive as the blocksize approaches the limit of 500 kB."

The size of the queue of unconfirmed transactions waiting for inclusion is known by your client but can't be viewed. It can be seen here: http://bitcoincharts.com/bitcoin/txlist/. At this immediate moment, there are 2763 unconfirmed transactions (1454847 bytes). That would seem to me to be larger than the maximum blocksize indicated above, so the above rule about higher transaction fees may be kicking in on the miner's side for inclusion of transactions (but the client isn't prompting).