This isn't a long-term deal per se; instead it's just a way to cheaply buy out Capps' first year of arbitration eligibility. Capps gets $750,000 this year ($500,000 plus a $250,000 signing bonus), then gets $2.35 million next year. He would've made $435,000 this year without the bonus, so basically this contract amounts to a $2.6 million deal for 2009, signed a year in advance.
Most relievers don't become closers as early in their careers as Capps did, so it's tough to say how much he would've made in arbitration next year. Among current players, Chad Cordero of the Nationals may be the most comparable in that he went into his first arbitration hearing (last offseason) as a pretty good, proven closer. He won his case and made $4.15 million. Relievers are volatile, but Capps is young enough and good enough that gambling $2.6 million now to keep from potentially paying $4 million or so for 2009 seems like a pretty good idea for the Pirates.
Capps will still have two more years of arbitration eligibility after 2009, so it isn't like he'll be a free agent when the contract is over.