Where are you located?
If you are in the UK, there are a few services that you could use to check the address based on their post code (zip code)
I have it in my store, they enter their postcode, et voila a list of address options appear and they click their address, it put this into your cart nicely formatted.
This also has advantages that you dont get people writing their address as
MY hOUSe
Some street
A CITy
some KOunTy
But granted it doesnt check if its right, I wouldnt worry too much about validating it personally.
<edited to say> I just read it again, and id bet you are in the States, in which case this isnt really an option as most people that side of the pond dont know their full zip code.



