It is best to graduate in fall as you'll have few months to get hired and prepare the H1 petition to be filed on April 1. I think you'll have only one day to file your petition so you would like not to take any chances.

If you graduate in May, you'll miss H1 filing for 2008. Assuming your OPT starts right away after you graduate in May, you can work for one year and can file H1 for 2009 on April 1, 2009. But the problem is, your OPT will expire after one year and you'll have to join school again to maintain status until October 1, 2009 which is the first day you can start working.

I'm not sure if the letter stating that you'll graduate in May will work. I know that a copy of your diploma is needed. If the letter works, you can file in April 2008 which can be the best thing for you right now. If not, it seems you are all set to file for 2009.

No it depends whether you want to:

- Graduate in May 2008, work for 1 year on OPT, file H1 on April 2009, stop working after OPT expires, take risk and try to maintain status until Oct 1, 2009.

OR

- Graduate in fall 2008, start working on OPT, file H1 on April 2009, continue working on OPT until Oct 1, 2009 when your F1 changes to H1.

Good Luck!