Hello,
I have been on H1-B visa since Oct 05, and before that I was on F-1 for about a year or so. I am participating in my employer's stock purchase plan and the brokerage firm that administers this is asking me to fill out a form W-9 or a form W-8BEN. Which form should I be filling out? Am I to consider myself a resident for tax purposes and fill out a W-9 or should I fill out a W-8BEN considering myself an alien?
Please advise.

Thanks!

You should fill out W-9. If you change your status and become a nonresident alien, then you should provide a W-8BEN to the brokerage firm at that time.