Dave has already laid out his full explanation of why this seems like an empty rumor, so you should go read that. It's a solid, sensible post, and there's no point in my repeating it when you can just go over there. Go away! Go away from my website!
There is one additional little matter that he didn't really bring up, though, and that's this: Adrian Beltre is better than Jason Bay, will cost less than Jason Bay, and would fill a need, and the team is letting him go.
Think about it. Right now, the team needs a starting pitcher, a third baseman, a first baseman, and a DH. Bay could be jammed in there with a little creativity, but Beltre would fit no problem. While Beltre would block Tui, Bay would block Saunders, and the latter would be a less manageable situation since room for Tui could be cleared by dealing Jose Lopez, which the team seems to want to do anyway.
Beltre's also looking at less of a payday. Bay has reportedly already turned down a $60m/4yr offer. Even if that's around where he ends up, do you think Beltre's looking at anything comparable? Not coming off an eight-homer season he's not. Beltre will be compensated well, but not as well as Bay. The market is going to love Bay's 30+ home run power and 119 RBI a lot more than it's going to like Beltre's hidden value.
Finally, there's the matter of Beltre being the better overall player. This one might be hard for some of you to believe, but if you can look past Beltre's poor offensive 2009 and concentrate on how each player projects going forward, then the gap becomes more apparent.
The 2010 wOBA is adjusted for park and loosely based on a 5/4/3 weighting system. The defensive projections are fairly conservative, as there's reason to believe Beltre is more like a +15 and Bay is more like a -15, but it's better to err against your argument than in favor of it.
The difference between .380 and .330 over 600 plate appearances is about 25 runs. That's pretty much erased by the defensive gap before you even take into consideration that Beltre plays a premium position and Bay does not. Plugging these figures into the WAR formula yields a gap of about five runs in favor of Beltre, and adjusting the defensive numbers could take that up to a full win, if not more (neither seems to be an exceptional baserunner). Basically, to believe that Jason Bay is better than Adrian Beltre, you have to really, really believe that Beltre's 2009 offense is more representative than his previous track record. And even then, the defense might make up the whole difference anyway.
It would be one thing if we still had a stupid front office. Stupid front offices don't act rationally, and as such can be drawn to things like RBI and hollow labels. But that's not the Mariners. Not anymore. The Mariners as we know them now are intelligent, so intelligent that they probably already thought of all this months ago. These guys know that Jason Bay isn't as good as people think, and that Adrian Beltre isn't as bad as people think, and that the latter is most probably better than the former in terms of overall value.
And yet they're still likely letting Beltre go. Why? Because they can't really afford to pay him and still do what they want to do this winter. They have the room to bring him back, but that would make things more difficult on them. And if they don't want to spend $X on Adrian Beltre, I can't imagine they'll want to spend >$X on an inferior player, particularly one who would block a good prospect.
Forget any angles about how Bay wants to play here and Beltre wants to get out. Money makes up people's minds. Bay wouldn't sign here if we didn't offer enough, and Beltre would if we did. That's not a factor. What matters is that Adrian Beltre is both a more valuable and more affordable asset than Jason Bay, so if the team's prepared to let Beltre walk, then Bay just doesn't make any sense.