Should I file jointly if (non-legally) separated in 2007 to save him $? Will IRS pursue me if he defaultsloan

We have 3 children, 2 with disabilities; I have sole custody per private notarized Separation Agreement. Married 13 yrs; he's self-employed and never pays his quarterly taxes--this is usually the only tax he ends up signing a loan with the IRS for, each year--b/w 3500-6500. We've been separated for 2 years, but lived under the same roof (I don't recommend this!) until the kids and I moved out of state in Jan 2008. So, for 2007, he wants to file jointly so that he'll owe much less...but I'm afraid that if he defaults on the IRS loan, if I file jointly I am automatically on the loan?, and can't pay his loan...Also, I'm afraid that I will not get any of the Tax Relief/Economic stimulus check...I don't think I have to file at all, since I had no income. I'm not opposed to filing jointly to help him, but don't want it to bite me...! I appreciate any advice/insight you can provide.