I know you've cited moving to Exchange 2010, but FYI: Exchange 2013 (ie. the version that has just released) has built-in support for DLP, but it's currently only available for e-mail (including Outlook 2013). That will expand in the near term. But Symantec has a much larger offering.
If you're planning to use the DLP feature built into Exchange 2013, I will point out that the default "out of box" classification rules (ie. the rules which are responsible for finding sensitive information like credit card numbers, social security numbers, etc) have poor accuracy in my experience. We found this for ourselves, and there are obvious & unfortunate detection misses. We're now using a drop-in replacement from Nucleuz.