Here's his contract and incentives:
Base: $1.5M
Opening Day 25-man: $1.0M
Games (50, 55, 60, 65, 70): $0.5M ea - if all $2.5M
Games on 25-man (30, 60, 90, 120, 150): $0.5M ea - if all $2.5M
He's earned the first incentive, and the first two from the Games on 25-man Roster, so he's at $3.5M now. Through 65 games, he's pitched in 24, so he's on pace for 60 even. Let's assume that if he's traded, as a closer his usage goes up a little and he pitches in 65 games. That means he'd earn another $3.5M the rest of the season. I'd be willing to bet the Sox would trade him, if they wanted to do so, with their responsibility being the base + incentives earned to date, so the receiving team would only be on the hook for his remaining incentives, which would total $3.5M if my assumption is met.
In that scenario, the receiving team would get an established late game reliever at little to no risk. If he gets hurt, they owe him close to nothing because incentives won't be met. I'd say that makes him an attractive solution to their needs.
Re: draft compensation if he's retained and offered arb. There's a club option at the same base, with clauses for incentives and what happens if the option is declined. The incentive clause says that if the option is picked up, in 2010 he will earn any incentives that were unearned in 2009. This is pretty big, IMO. This means that if he finishes out the year healthy, he'll be dirt cheap next year. He'll have earned his Opening Day 25-man roster bonus for $1.0M, he'll have already earned the Game Appearance and Games on 25-man Roster incentives to the max of $2.5M (in my assumption, the 70 Game Appearances would still be unrealized, meaning a $0.5M bonus would still be out there in 2010). They have their closer for a base of $1.5M with no (or almost no) incentives. Very attractive. If the option is not picked up, the player is released outright (no arb offer, therefore no draft compensation).
So when you total it up, a team trading for Saito would be getting their potential closer for a max of $3.5M this year and $2.0M next year, with total protection from the risk of injury if it occurs before they pick up the option.
I'm sure that will garner some interest.