One of the rumors floating around is that Texas GM John Hart asked for Mike Cameron in exchange for Alfonso Soriano. It's unclear if other players were involved, but the Mets turned it down.
Do you trade Cameron for Soriano straight up?
Such a trade would hurt the Mets defensively in the outfield, regardless of who they replace Cameron with out there (Valent, Diaz, etc.). Soriano is a slightly below-average defensive second baseman, so the Mets probably won't be losing much if anything there, as he'd be replacing the sucktacular amalgam of Miguel Cairo/Kaz Matsui, both of whom are average with the glove.
Soriano, despite his horrendous home/road splits in Texas, would represent a substantial offensive upgrade at second, as he is still one of the few best hitters at his position and Cairo has been just about the worst. Soriano is fourth amongst second basemen in OPS at .860, trailing only Brian Roberts (.968), Chase Utley (.924) and Jeff Kent (.898).
Does Soriano plus Cameron's replacement in right field represent an overall upgrade over Miguel Cairo and Cameron? Defensively it is almost assuredly a "no". Offensively you would have to say yes, even if it's only Eric Valent who replaces Cammy. Valent is hitting .303/.404/.517 at Norfolk, playing some outfield and first-base, including a 24/26 BB/K ratio.