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Yes the tax will be due by April 15 of 2010.
You will report the gain on Form 4797.
Here are links to the 2008 instructions & Form 4797:
If you have any further questions on this topic be sure and get back to me.
The question of when taxes are due is a tricky one. Ultimately all taxes due will be due by the filing date.
Keep in mind that estimated payments are required, especially if the tax liability is affected as a result of income increases.
If you do not make the estimated payments in a timely manner then you could be subject to underpayment penalties.
I cannot advise you not to make those payments but you have to weigh the potential gains you might accrue against the potential penalties & interest for underpayment.
Here is a link to an IRS Tax Topic that will assist you in determining what your potential underpayment penalties might be:
If you have further questions on this topic, get back to me.