Send bitcoins using just a pub and private key
EDIT: I recalled that Wiki: Sweepprivkey proposal, is actually exactly what you are asking for, bitcointalk.org: Proposed RPC command: sweepprivkey. The latest news about the work in progress from jarpiain, dated October 15.


  This is functional in the sense that I tested it succesfully on the
  real block chain. It's not yet robust enough for serious use. In
  particular, there are race conditions for accessing addrblock.dat.
  
  I'm going to implement some of the sweepprivkey options and copy the
  transaction fee logic from CWallet::CreateTransaction() this weekend
  (I think it's best to keep the fee setting in one place, the -paytxfee
  option).


MtGox has the option to fund account with private key, and it understands several different formats, not only the standart one, e.g. it accepts mini format used by Casascius physical coins, MtGox adds redeemer for private keys including Casascius Physical Bitcoins

But if you want to send the transaction from your own computer then you need a software that would create a proper transaction using the public and private key, bitcoin or bitcoind can't do that without importing keys into wallet first, importing could be done with Pywallet or hopefully soon-to-be-official wallet export/import bitcoin patch, but make sure to backup your wallet first.