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.