Many folks on Twitter have explained this very well, so I have quoted their guidance here read more...
and on Twitterific for iPhone, the retreat function automatically inserts "(via @...)" instead of RT
Powertwitter (for Firefox) adds RT instead of via @ - so I guess it's all up to the user.
Using "via" uses up 1 more precious character than "RT", so that's also a consideration.
That's not true... RT is for if you're being lazy and you just want to tweet someone elses post. Via @ it for if you want to say something about then repost it (Example: "Cocopops149: Hey check out my YouTube channel http://bit.ly/ayFG7r," I would post ,"Stranger123: I lol'd so hard when I saw this YT channel http://bit.ly/ayFG7r via @ Cocopops149,")