Please verify the notice you received - does the IRS indicate that the mistake was made om their side or on your side?
When did you send your 2008 tax return?
Did you claim any Recovery rebate credit?
- line 70 form 1040 - http://www.irs.gov/pub/irs-prior/f1040--2008.pdf
If they admitted the mistake - you should already receive additional $600.
There is no other way to find out - but to contact the IRS directly.
You may call 1-800-829-1040,
or just walk-in your local IRS office - please find here convenient location in your state - http://www.irs.gov/localcontacts/article/0,,id=98337,00.html
Sorry - that is the only way to solve your problem.