The Health Protection Agency carried out a seven year study of needle stick injuries within the medical professions, of the 2140 cases examined, there were no reports of HIV or Hep B infection but markers for Hep C developed in nine people. So you see the chances of getting a Blood Bourne Virus (BBV) are very low.
In saying that, the first thing you should do (ok second thing after swearing) if you do get a needle stick injury is to squeeze the injured part and make it bleed then wash the effected part (we carry alcohol gel in our caching bag), and in the next couple of days see your GP.