Based on the reports of quite a few Surly Pugsley owners on MTBR.com there seems to be a problem with a fair number of Pugsleys made in the last few years. Cracks are appearing where the seatstays attach to the seattube and where the cahinstays attach to the BB. The cracks seem to all be on the non-drive side of the bike.
You’ve got a 3yr warranty on your Surly so as long as you keep an eye on this and report it within the warranty period Surly will give you a new frame. Not everybody is experiencing this problem so I wouldn’t panic, but it’s wide spread enough to warrant regular inspection of the affected areas.
Both of my 2009 Pugsleys have been used hard and don’t show any cracking. Surly has changed the way these parts of the bike are attached to each other for 2013 so the problem should be solved.