Well... to be fair, Jake Sheilds is a 185er... which is what he fought at last night and I'm pretty sure where he intends to stay. Sure he can to 170, but his fram is much more suited for MW. Dan Henerson is also a natural 185er, he is so awesome that he's able to complete at 205 but make no mistake, he's built for 185. Take a look and he and Sonnen standing together some time, Sonnen is a far larger man. Point being, Henderson losing to Jake is just one middle weight losing to another.
As far as challenging for the 205 title, well that's just something that Dan has earned simply based on his career and the streak he is currently on. Plus, it's an easy sell for the UFC, and we all know he has just as good a chance of beating Jon as anyone else... which is to say very little in all honesty.