I got my 10 year LPR card today. I've had the LPR for two years before. I got married on Dec 28, 2003. How soon before I can apply for US passport?

You were LPR when you married? If so then 3 years from your wedding anniversary, you can naturalize, as long as you have not had periods out side the country for longer than 6 months.

After you naturalize you can get an American passport.

See: http://www.uscis.gov/portal/site/uscis/men...00045f3d6a1RCRD

got married on dec 28, 2003, I was on F1 visa. got the green card on feb 7, 2005. so wait till feb 7, 2008? or what?

You have the correct idea, 3 years from the date on the green card if still married to original petitioner, 5 years if not married.

This guide is useful and explains the requirements. http://www.uscis.gov/files/article/M-476.pdf

when do I apply for it? On Feb 7 2008 or a 90 days prior to it?

you have the right to apply 90 days prior to your third year anniversary, that means you can apply sometimes on Nov 2007.
Good luck

sounds good. can someone else confirm this as well?

Yes, you can easily confirm it by glancing at the Guide to Naturalization at uscis.gov (see the waivers section). I highly recommend that you use that source to confirm all rumors about applying.
There are also comperhensive links and information in the pinned post at the top of this forum.
And use a date calculator (find one online) to verify your application date.