15 years is a long ARM... I figure in that time, we'll have either paid it off, moved, gone bankrupt and/or I'll be dead
After 15 years, there'd be about $289,000 remaining in principal. How much the rate can go up is capped, so I think it's around 8.65%. If you paid 8.65% on the remaining principal, assuming taxes and insurance the same (which they won't be), it'd be a payment of about $3,050 per month today versus $3,750 in 15 years. Not good, but won't be the end of the world.
An ARM now though...
Interests rates are going up