Refinancing out of ARM loans is the right thing to do especially if you plan to stay in the home. The most advantageous structure (returning to 80/20 or 1 loan) would have to be determined by a competent mortgage provider after seeing all the data.
One thing to be aware of in making that decision: PMI is now, in fact, tax deductible for the first time in our history (Congress passed the bill and President Bush signed it into law end of December, 2006). Any refinance that only refinances the original amount (plus costs) is eligible. So should the best structure turn out to be a one loan with PMI, you want to get it done inside 2007. You'll be able to deduct the PMI premiums for as long as you hold the loan.
Tax deductibility for PMI is also for purchases completed in 2007 as well. I wrote a post on my blog about all the other qualifying conditions...
click here to read.
Your biggest problem in refinancing may come in being "upside down" or "underwater" on your home's value. Property values are dropping all over the country and should your new loan balance with costs rolled in be more than the home is worth...well, no refinance. Another good reason to get it done, if it's doable, as soon as possible.
Hope this helps...

Rob K. Blake
The Mortgage Insider