Rosenthal's source suggests that Oswalt is not yet in mid-season form, but Jon Heyman of CBS Sports (and his source) suggests Oswalt will likely be back "within a month."
The rumors follow an off-season in which Roy Oswalt was particularly stringent regarding the teams he would be willing to join. Now, the same candidates from the off-season are the ones mentioned to have Oswalt in mind.
The Philadelphia Phillies were Oswalt's last team, and it appeared in the off-season that they weren't going to have room in the rotation. But Vance Worley's recent elbow concerns have probably made the Phillies explore other options.
The Boston Red Sox seemed like the best fit for Oswalt in the off-season, but he didn't appear particularly keen on pitching a full season that far away from home. For a half-season, though, he apparently will be open to the idea.