Well if you haven't; you will very shortly. Washington DC residents were sent last week their 2010 Proposed Property Tax Assessments. 
What you should know:
-You need to make sure it's correct. A favorite saying: "I think Tax assessors sit in a windowless room and dream up numbers." They're NOT always correct.
-Make sure your Home Deduction is listed. If you live in DC and this property is your Principle residence; you should qualify for this. The Homestead Tax deduction was increased from $64,000 to $67,500. This will lower your tax bill by hundreds of dollars.
-If it's not right - Fight it. You must first file for the First Level of Appeal by APRIL 1st, 2009. If you miss this deadline - it's all over.
So how do you know if it's not right? Well if you just bought your home recently, it should be close to that amount (less your Homestead exemption). If you bought years ago you will need to do some homework. Find the prices of homes that are like yours, which have sold recently. If you next door neighbor (or condo across the hall), is similar to yours and they bought their place 3 months ago for $400,000 - your placed shouldn't be assessed at $450,000!
Need help finding this information? Ask a Realtor.
and as always, for tax advise, seek out a tax professional...

Contact James Downing if you are selling or buying. Cell: 703.244.3971
Licensed in Washington DC, Virginia & Maryland.
Coldwell Banker Residential Brokerage - Chevy Chase
5028 Wisconsin Ave NW Suite 100 * Washington DC 20016
202-351-5800 Office
Hey, James! Good advice! The tax bills should be a lot lower this year.