How many unspent transaction outputs are there? I was wondering the other day how many unspent transaction outputs there are ?

If you just keep track of the unspent transaction outputs you could import private keys, know its balance  and perform spends. How much data is this compared to the full blockchain ?

The biggest downside is that you would not have the full transaction list for a private key/address - just the transactions that you can use for future transaction inputs.
As of block 177778 (Apr 29, 2012 6:17:33 PM):


2944702 transactions
6904591 transaction outputs
5376092 transaction inputs
1528499 unredeemed outputs left
642432 addresses with non-zero balance