I find the concept of open sourcing a saas product interesting.

On one hand I can understand the concept that withholding the source code gives a major competitive advantage, and of course it may lead to someone creating his/her own clone (think of identi.ca for twitter). 

But there is something that can also be considered:
 stackexchange concept embraces a lot of generatives that would make it better than free/open source and thus valuable by itself (http://www.kk.org/thetechnium/archives/2008/01/better_than_fre.php) 

 - Immediacy, it's going to be the first service of it's kind available and as a saas setup time is (should be) minimal (compared to installing the open source version on your own server).
 - Personalization, well that's the whole goal of stackexchange 
 - Interpretation, this essentially boils down that stackexchange would also be the best group to provide commercial support. Essentially saying you can deploy the software on your server for free (and thus provide you the control) but technical support can be bought for your piece of mind (plus other services such as consulting (or certification for onsite support), interpretation/analysis of the data could be considered ie: how to improve/increase interaction amongst the users of a client).
 - Authenticity, only stackexchange by the creators of stackoverflow can give this.
 - Accessibility, a saas is available everywhere (though an API and mobile version would be nice), an internally deployed version might not be.
 - Embodiment and Patronage might not apply to stackexchange as pure saas play
 - Findability, stackexchange is a brand and it can promote itself more than the open sourced version and therefore get more clients. Also stackexchange will profit from the relation to stackoverflow (this also enforces the authenticity) 

Regardless if stackexchange will have an open sourced version or not, it does seem to have all the ingredients for success.

Personally I believe that an open sourced version would help stackexchange to be more successful as it reinforces a lot of the points mentioned above and it could open the last two point Embodiment (local stack exchange user groups) and Patronage (users giving back by evangelizing and contributing code / plugins / bug fixes etc)