Thanks for your question.
If you cannot pay your 2009 tax amount owed, you should go ahead and file your return by today. An extension of time to file will not help since you will owe more penalty and interest when you actually file the return.
The phones at the IRS will be very busy today, so it would be better to call later. After you file your return today, you should wait for the return to be processed which can take from 2-3 weeks if you e-file or 4-6 weeks if you file a paper return. Then, you can call the IRS and ask that your installment agreement be restructured to include your 2009 balance. They may be able to add your 2009 balance to your installment agreement or they can instruct you what to do.
You should consider either increasing your withholding allowance on Form W-4 or making estimated tax payments in order to avoid owing large amount of taxes in later years.
For reference, see http://www.irs.gov/taxtopics/tc202.html
Hope this helps,
That sentence from your notices will apply if you do *nothing* to deal with the new tax balance owed for 2009. However, if you follow my instructions above and ask the IRS for a restructured installment plan, this should not happen.
From the IRS web site: "If you already have an installment agreement from a previous tax debt and your financial situation has changed, we may be able to modify or restructure your installment agreement to include additional amounts owed into one agreement. Additionally, a Collection Information Statement may have to be completed to further illustrate your financial situation. If an installment agreement is modified, reinstated or restructured, a $45 user fee may be charged." http://www.irs.gov/businesses/small/article/0,,id=156495,00.html