Search engines completely ignore viewstate, however they do have certain limit on how much of the page they will download, in general you should work with 100KB for sake of all engines not just the big ones (according to test back in 2006 it was Yahoo - 210KB, Google - 520KB, MSN - 1020KB) while average DNN page takes around 40-50KB  ... so if you have very long viewstate in front of your content, mixed with inline CSS, Javascript and more controls it may move part of the content outside indexing range which is the SEO problem (both indexing and ranking) you are talking about.

That being said there's simple logical solution ... you can't really persuade module developers to optimize thoroughly (hell 50% of them even inject huge inline javascript), but you can move viewstate below the content, then it won't matter if its 10KB or 500KB it will just slowdown page loading, but won't affect indexing/ranking at all :). I'm using PageBlaster with its replacement rules to do this, however I do wonder why DNN adds it on top in the first place.