from the IRS website
http://www.irs.gov/publications/p505/ch02.html#d0e4865
General Rule
You must pay estimated tax for 2005 if both of the following apply.
1.You expect to owe at least $1,000 in tax for 2005, after subtracting your withholding and credits, and
2.You expect your withholding and credits to be less than the smaller of:
1. 90% of the tax to be shown on your 2005 tax return, or
2. 100% of the tax shown on your 2004 tax return. Your 2004 tax return must cover all 12 months.
Tip
If all your income will be subject to income tax withholding, you probably do not need to pay estimated tax.