Maybe I am too naive.. but I was under the impression that you need to get H1B first and apply for green card. Can we file for a green card on OPT?

Yes you can file EB2 on OPT. In fact I have seen people file EB2 NIW while still doing PhD. However, the downside is that if it gets rejected you will be out of status. But if you are in H1B and your I140 gets rejected, you will still be in status and can reapply.

Why most people do not apply EB2 while on OPT ?

Answer is simple..EB2 or any green card process is complex and may take extra time.
These days labor certification takes about 4-6 months if no audit and if audited takes another 3-4 months extra.

So, when you're in OPT (you've have 1 year legal status). Now when you apply EB2 while on OPT and your process takes more than a year then you're OUT OF STATUS !

So, it's better you apply H1b then apply for EB2.

You've following benefits:
-you'll be in status for 3 years
-Easy to show that your company is paying you salary for the same job (tax return etc)
-if Labor Certification is rejected you can apply again

so I suggest applying after H1b.

Yes you may start Green Card now and apply H1b in April 2013.

Green card is for future employment anyways !

Hope that helps.

 Thanks Sparty and guy_tensed for those answers. New stuff for me.

guchcha