Taxes in Prince William are due on June 5th and December 5th, for June 30th and December 31st taxes (1st and 2nd half). Depending on when you settle, the portion paid by the seller will be prorated if you are buying after the first or second half taxes have been paid.
In other words, if you were to close on October 30, 2007, the seller will not have paid his second half taxes because they are not yet due. You will be receiving a second half tax bill as the new owner prior to December 5. The title company will determine the seller's responsibility and credit you that portion of taxes due from July 1 through October 30th. Then you will be assessed the remainder of the second half taxes due from October 30th (date of possession) through December 31st. This will appear on your HUD-1 as a charge.
Your lender will likely escrow anywhere from 3 to 9 months of taxes depending on when you are going to settle. Try to find out from your loan officer ahead of time how much this will be. Taxes in Prince William are assessed at a base rate of $.787 per $100. You may find precise calculation depending on the township or additional levies at this site...pwcgov.org
You may search any property's tax assessment by linking to LandRover on the county's website and searching by parcel or address. Also, if you question the county's assessment of your property (which is not uncommon considering the fluctuation in the market over the last 12 months), you may go to the site to appeal it...also on the county's website within the real estate tax assessment section.
Best of luck!