This is a procedural move, one that will likely result in Feldman staying in the organization, and not terribly surprising.
Feldman is out of options,* which means he has to clear waivers to be sent to the minors. With his 30 day rehab stint expiring, the Rangers either had to leave him on the disabled list, activated him and put him on the 25 man roster, or activate him and put him on waivers to send him to the minors.
* I thought earlier that Feldman didn't use his third option, but I was wrong.
Given his contract -- $4.4 million this year and $6.5 million in 2012, with a 2013 option -- and Feldman's struggles last year, it is unlikely that any team will claim him. If he goes unclaimed, he will be outrighted to AAA, will not be on the 40 man roster, and will toil at Round Rock until the Rangers decide to bring him back to the majors. If he does get claimed, I think the Rangers would privately breathe a sigh of relief that his contract is on the books, while publicly express their regret that Feldman is no longer here.
While there was talk that Feldman could replace Derek Holland or Alexi Ogando in the rotation, or move into the bullpen as a long man, the Rangers have apparently decided to go ahead and keep him in the minors for the time being. The Rangers can purchase his contract at any point and bring him back to the majors, and he'll presumably be up when rosters expand on September 1, if not before then.
UPDATE -- Jeff Wilson mentioned that Feldman can refuse an outright assignment. I thought that was incorrect, but in going back and checking the service time, it appears that Feldman probably does have 5 years major league service time, which would allow him to reject an outright assignment.
However, I have to believe that the Rangers and Feldman had already agreed he would accept an outright assignment. Otherwise, I don't think they would have made this move.