Edit: This has been finally won't-fixed with Gecko 17.0.1 after hitting the release channel with 17.0, thus the following doesn't apply to any current builds. However, freezing the build date to Gecko/20100101 is now being performed for SeaMonkey as well!
Thus, expect pages to break which rely on parsing Gecko/YYYYMMDD from the user-agent string, which will be replaced by the Gecko release (i.e., Gecko/13.0 SeaMonkey/2.10a1). According to a quick test as reported in a spread-sheet attached to the bug report, the most frequently visited sites seem to still work properly with the modified UA string.
No other changes are performed at this time, on the long run though it's still envisioned to remove the "rv:xx.xx" component in the comment section after that moved into the Gecko token.
To obtain the build-date information on a web site, you'll then have to use navigator.buildID in your JavaScript code (as is already the case with Firefox builds which have a frozen date string in the Gecko token, neither SeaMonkey nor Thunderbird followed that move).