It doesn't appear to be too serious.
From the team's official homepage:
Halladay traveled with the team to Boston for its current series against the Red Sox, but the right-hander continued to experience soreness in his forearm -- prompting the team to have him undergo an MRI exam on Monday. The results showed inflammation in Halladay's arm and the Blue Jays had him fly back to Toronto to see the team doctors for further tests on Tuesday.
As of right now, Toronto doesn't expect the injury to keep Halladay on the shelf for more than just one appearance.