If you are confused about who Chris Seddon is you are probably not alone! Seddon is a 26-year-old lefty who has spent most of his time this season in Tacoma's rotation and has been hovering around Triple-A since 2008. He has been decently successful there and should see a boost returning to the bullpen. Most importantly, he is not Sean White!
Uninterestingly, Seddon is not on the 40-man roster so he would have to be added there first. There are a couple open slots so nobody has to get bumped, but given that I assume Seddon's stay is limited by how long it finally takes Erik Bedard to come off the DL, this seems like a weird use of a 40-man slot.