Last year I had the third overall pick in Dewey and chose LaDanian Tomlinson (it's a keeper league so some of the top picks were unavailable), passing up Johnson and DeAngelo Williams, among others (my post is here). That choice pretty much derailed my entire season, and I am hoping that I do not similarly screw up this year with the #6 pick. I'm looking at Calvin Johnson and Shonn Greene (is Greene a keeper?) as possibilities, though if Frank Gore and/or Steven Jackson is available, I'll have to consider them.
There's a good article in last week's Sports Illustrated about the phenomenon of running backs with increasingly short careers. The star running backs really do have a short window -- it seems like 4 or 5 years is now the most you can expect for an RB to be at the top of the game. The article makes the point that teams view running backs as expendable/replaceable, as reflected by the position having one of the lowest average salaries of all positions.
This is a pretty amazing change. In the 1980's, Walter Payton was the second biggest star (behind Montana), and in the 1990's Emmitt Smith was tops (above every quarterback, in my opinion), but now the super running backs cannot have anywhere near Payton's or Smith's longevity to become the league's most acclaimed players.
One of the reasons for this phenomenon, I'd guess, is that defensive players are bigger and stronger -- and hit the running backs harder -- than they used to. The SI article does not really examine this, though it does say that RB's are more prone to injury.
In terms of the Redskins RB situation, it's uncertain that we'll have a true primary back this year; it sounds like Portis will continue to get the most carries (if he stays healthy), but Larry Johnson is getting a bunch of reps in the preseason games. My prediction is that no Skins RB will get more than 750 yards for the year.