Per Freddie Mac, these are the mortgage interest rates as of June 19, 2008:
• 30-year: 6.42%
• 15-year: 6.02%
• 1-year ARM: 5.19%
The difference between the 15-year and the ARM is .83%. On a $100000 mortgage that would amount to a savings of approximately $830 per year, or around $69 per month if you get the ARM. Of course, there's the risk that all rates will increase and the ARM rate will go higher than the current fixed rate.