I think Morneau's highest value would come in the trade deadline next season.
Of course if you get an offer you can't refuse this off-season then trade him, but if he continues to hit well (I don't see why he won't) He could be back to a .850+ OPS, 25+ HR guy next season, an impact bat like that at the deadline next season could potentially bring back a Beltran like package.
Also, I think the Twins have a decent chance to compete in 2013, I don't think its a coincidence that once Morneau started hitting better this year the team started playing better overall.
I'd much rather trade Span this off-season since he could bring in a bigger haul, you can find at bats for Parmelee at RF and DH in that scenario.
Lastly, I like Morneau and he is one of my favorite Twins players ever, I'd personally hate to see him traded, especially for a salary dump type reason.