Apparently, the deal will be for one year. As yet, no other contract details are available aside from the fact that Rasmus will earn north of 4MM.
A 40-man roster place would have to be cleared for either Rasmus or Ryan Vogelsong to be added. I doubt either would agree to a minor-league deal.
SNEAKY BATGUY UPDATE: Looks like the deal for Rasmus is for $8M. Carlos Corporan has been designated for assignment to make room on the 40 man. Corp had recently agreed on a $975K, 1 year deal with the Astros to avoid arbitration.