There are a couple versions of this deal. I am not sure if either are realistic, but they are worth exploring.
Brewers trade SS J.J. Hardy to the Mariners for SS Yuniesky Betancourt, SP Jarrod Washburn, RP J.J. Putz, and RP Mark Lowe
Why it makes sense for the Mariners: The Mariners need offense and defense, and they have a logjam in their pitching staff. While the loss of Putz would be noticed, Washburn and Lowe's combined 2008 production is replaceable from within, and Hardy would be a major upgrade over Betancourt.
Why it makes sense for the Brewers: With Ben Sheets and CC Sabathia hitting the free agent market (and Salomon Torres unexpectedly retiring), Milwaukee has all sorts of pitching holes. Washburn and Putz would fill a couple of them. Betancourt would be a downgrade at shortstop, but Milwaukee thinks top prospect Alcides Escobar is nearly ready for the majors anyway. If he is ready now, Betancourt could slide over to second, and Milwaukee's infield defense would be better than last year's. All of those are fairly short-term fixes though, so Mark Lowe is a piece that can stay in the organization for years to come, given that they are giving up a young shortstop that could be a Mariner for years to come.
Problems with the deal: The legitimacy of this deal is contingent on what the Brewers think of Betancourt and Lowe. Do they still think Yuni is a great fielder, and could be a decent hitter? Do they think Mark Lowe can re-gain the dominance he flashed when he first came up? If they say yes to both, this deal should be tempting. If they say no to both (as I would), they will probably look for better deals.
I'm not sure how Milwaukee would react to trade posed above. If they rejected it, I would make the follwing offer:
Brewers trade 2B Rickie Weeks and SS J.J. Hardy to the Mariners for SS Yuniesky Betancourt, SP Erik Bedard, RP J.J. Putz, OF Jeremy Reed, and RP Mark Lowe.
Why it makes sense for the Mariners: It's pretty much the same as above. The Mariners need offense and defense. Rickie Weeks could be the DH (as the M's try to move him to the outfield), and J.J. Hardy becomes the every-day shortstop. They M's also alleviate their pitching logjam with this deal.
Why it makes sense for the Brewers: If Bedard and Putz return to All-Star form, they more than offset the loss of J.J. Hardy, especially if Alcides Escobar is ready for the majors. Even if Escobar isn't ready though, Betancourt can play shortstop and utility man Bill Hall could take over at second. The bench would not be left bare thanks to Jeremy Reed, who is a decent pinch-hitting option and solid fourth outfielder.
Problems with the deal: How healthy do the Brewers think Putz and Bedard are, and how much are the Brewers thinking only about next year? This would be a risky trade with the possibility for big immediate returns. If they like Betancourt and Lowe enough as long-term options, this deal would look more attractive to them.
The Mariners should aggressively pursue a J.J. Hardy trade. If the Brewers are intrigued by J.J. Putz, Erik Bedard, and Jarrod Washburn, there may be a deal to be made. Hardy would be an upgrade offensively and defensively, and he is also under team control for a few years before he is even eligible for free agency. Beltre and Hardy would be oen of the best left diamond combos in all of baseball.