My case is

Jan '06 - May '06 = F1
Jun '06 - Sep '06 = OPT
Oct '06 - Dec '06 = H1

I conclude that with this status, I need to file as a non-resident. Now I have been contributing to 401k since Aug '06 so filing as a non-resident I can still keep that tax benefit, right? Also, the social security and medicare tax started to be deducted from my paycheck a month later i.e Nov (instead of Oct when my H1 started) so in the tax return do I need to account for that somehow?

Suggest you get an accountant :confused:

You do NOT need to account for the Social Security and Medicare taxes; that responsibility rests with the employer.

You will probably file as a non-resident alien, unless you are married, in which case you can file jointly as resident aliens.