An open source library and full product have been developed for transparently splitting large tables across several databases for performance, similar to database table partitioning.  It is all handled along with the Linq to SQL framework, and looks pretty slick, I will be reviewing the product shortly.  It looks mostly transparent to the developer!  There are other capabilites worth a look.  This looks like it works for azure as well.

Here are some links: 

http://enzosqlshard.codeplex.com/
 

http://enzosqlbaseline.com 
 

https://scale.bluesyntax.net
 

I will be reviewing this and other Linq to SQL libraries soon.