With a pre-selected line, if you want to change its lineweight via the Ribbon, you should be able to simply change it on the Ribbon, but it screws up, and passes the wrong kind of unit to the Lineweight command. I could easily fix this, but I don't see the linewieght command for that tab anywhere in the cui. Shouldn't it be under Properties in the cui? I clicked Ribbon/Home-2D/Home - Properties/Row2/Ribbon Combo Box - Lineweight. Then, under Properties, I see Display, Appearance, Access, Advanced, Display, and Images. No Command. Where should I look for the command that's messing up? I know what it's doing wrong because I can see the error at the command line.
I looked at my Civil3D 2011 cui and it looks the same as the AutoCAD 2010 cui except for the key tip. I looked through the Support directory and found an additional file in C3D named AclLineWeightEngine.dbx. So I can say it's been fixed because it works in C3D. I came across another post on another problem in Acad2011 and support stated they were aware of that problem and "advised" them to upgrade to a newer version of the software because they were not going to fix it or put out any more updates for 2011. So you can figure for yourself where 2010 falls in the scope of priorities. It may apply to you case or not but thought you should know.
"Advised" to upgrade, eh? Hey, I gotta great way to generate new sales! With every new release, screw something up, then tell customers they need to upgrade to the NEXT latest version.
(Thursday is Cynicism Day for me - Monday is 'I HATE Mondays day, Wednesday is Hump Day, and Friday is Thank God it's Friday day)