Fixed or Adjustable Rate Mortgage??

It's Halloween 2010 and I reflect on the incredible financing oportunities of 4% 30 year fixed rates. Basically, if you borrow at this rate and have live in a 30% tax bracket, it's almost like free money. Over 30 years, how many real estate investment would not give at least a 4% rate of return. My only lament is that for my own personal residence, I originally got an adjustable rate mortgage for $230,000 starting at 8.9% in 1988 based on the 11th district cost of funds with a 1.5% margin. If I had kept that crummy loan with negative amortization instead of refinancing into a fixed rate mortgage, my rate would have adjusted down to around 2.3% at no cost to me.