It looks Seattle is likely to add some depth at the Forward position in June. Yes, this is outside of the transfer window, no I can't yet say who the player is. The player would be rotational (meaning not better than Jaqua/Montero/Seba), but considering that June is when the Open Cup proper starts, and the injuries that Sounders have faced this addition should help stave off our worries in the attack, but won't necessarily help the set piece situation.
Onto the DP trades in the past
Freddy Adu (not a DP but a exempted US MNT player) was dealt with Nick Rimando for Jay Nolly, a major allocation and future considerations. It was a rent a player because Adu was to turn 18 and eligible to transfer to Europe. RSL was able to buy low.
Chivas USA traded their slot in 2007 to New York Red Bulls for Amado Guevara.
Guevara was a decent player for NYRB, but not great. This year he is MVP quality, but that is three years after the deal.
He is a Honduran National though, so his value off the pitch must be considered.
Colorado Rapids traded their slot in 2008 to D.C. United for Christian Gómez for a two year period, which was later used to give Luciano Emilio a pay raise, taking him above the normal wage cap.
Gomez was a former MVP, so the value that the Rapids had is the highest we've seen.
D.C. United traded back their second slot to Colorado Rapids along with Ivan Guerrero in exchange for receiving Christian Gómez back.
In this deal you see that the DP slot has lost some value in the circles of MLS, likely due to a few failures (Denilson, Gallardo, Reyna and even Claudio Lopez who stayed in the league for 25% of the money).
The DP has lost some value over time, and it is notable that salaries of DPs range from 750k to 6,500k US $ or about 10,000 English Pounds a week to start. It will take more than just money to convince a player to come to MLS, as the league sits in around the 15-25 range for quality worldwide. So the DP targets would either be American/Canadian, post peak (over 30) or possibly a U-23 up and comer.
In the 2009 season with the CBA being negotiated, and the DP slot's final season in 2010 (RedBull is the only team with two that year), there may be more freedom to deal these slots, because a team that hasn't used it yet, may think that the future of the DP is likely to shift. Also since a DP is more likely to be a National Team player, in the 2009 to 2010 stretch they are quite likely to miss a few MLS matches.
MLS fans need to quit calling for Tevez, or Arshavin and understand that a player like Figo or Schevchenko is much more likely, no matter how regrettable. Goal.com just looked at "likely DPs" and notice they are all older. Too many clubs likely still see the DP as help for lagging attendance and not points in the table.
The good news is that in Seattle the ONLY reason to get a second DP would be to improve on an already good club, help overall depth and make a run for the silverware that the first year club has available (US Open Cup, Supporter's Shield, MLS Cup) and enter into contests in 2010 like the SuperLiga or CONCACAF Champions League.
That's the right way to make a personnel decision. What helps the team WIN. Let other teams worry about attendance.