I haven't posted my game review from the final preseason game yet, but it was pretty obvious why they might trade for a cornerback. CBs Pat Lee, Will Blackmon, and Brandon Underwood are all battling injuries, and CB Trevor Ford looked just awful, and he too was hurt in the last minute. WR Brett Swain played cornerback in the 4th quarter, and except for his tackling (which is something I'm sure he never practices), he was their best CB on the field. CB Jarrett Bush and CB Joe Porter are better, but they were both playing at safety, where depth is a problem too because of injuries to S Aaron Rouse and S Charlie Peprah. 

With depth an issue, they made a trade for CB/S Derrick Martin, according to Aaron Wilson at the National Football Post. Baltimore will receive a 6th or 7th round pick in 2010 in return. Although Baltimore's secondary isn't loaded with big name players, including former Packer CB Frank Walker, they were a very good unit last season, so he's been competing for playing time against some good players. Martin was injured all last season, and although he's played CB in the past, the Ravens were using him exclusively at safety this preseason. Baltimore Beatdown expected that Martin would win a spot on the 53 man roster, so he should be good. 

UPDATE: Great minds think alike. Mitchell and I were writing about this at the same time and his post got up before mine. Check out his post below.

UPDATE #2: It's a straight up trade of OL Tony Moll for Martin. Moll was the starting RT at the end of last season, filling in after RT Mark Tauscher went down with an injury, but he was always prone to the occasional lapse (i.e. missed assignment, holding penalty) so he was out of favor, and never even consider as Tauscher's replacement this season. He was probably going to be released, so acquiring a needed player for the secondary was a good move.