Odd. I find my Desire S’s GPS to be far more accurate than it really should be for a mobile… A lot of my riding’s under trees and it still seems to be accurate – within 10m most of the time, certainly rarely more than 50m
That said, some apps are better than others at filtering out bad results. I use everytrail pro and find it pretty good.
I would seriously consider whether there’s a problem with your phone…
(and thinking about it, this is my second Desire S – the first was similarly accurate)
edit – thinking about it more and having a quick look at my logged everytrail routes, it’s typically accurate to about 5m – the tracks certainly follow the actual trails on the map.