Having tried out the cleanup fixes from both Autodesk and Owen Wengerd, they both appear to work fine. Here are some points of comparison:

• Owen’s utility will work with any AutoCAD variant from 2007 on; Autodesk’s fix is currently restricted to Civil 3D 2009, 2010 and 2011. As this problem is definitely not confined to Civil 3D, and may need to be dealt with by non-Civil 3D users, that could be the dealbreaker right there.
• Owen’s can be installed by anyone by simply copying a file and loading it when needed or in the Startup Suite; Autodesk’s requires admin rights to either run an installer program or manual replacement of a program component, depending on the release.
• Owen’s loads and runs as the user requires; Autodesk’s runs automatically when opening and saving a drawing.
• Owen’s provides some information about what is getting cleaned up; Autodesk’s operates in total silence.
• Owen’s utility can take a while to scan through everything in a complex drawing; Autodesk’s appears to take no longer to open the drawing than normal. To give you some idea of the times involved, in one test in Civil 3D 2011, opening a blank ( but 2.2 MB!) drawing based on the Civil 3D template took 3.6 s with or without the fix; Owen’s cleanup took 0.7 s. In another test on an oldish PC with AutoCAD 2010, cleaning up a drawing with 2.8 MB of real content took Owen’s utility about 15 seconds.

For my purposes, Owen’s utility is what I need, because the users who need to clean up these drawings use AutoCAD, not Civil 3D. I’ve set up a batch process for these users, which opens each selected drawing, runs Owen’s utility and saves the drawing. However, I suggest Civil 3D users install the relevant updates and patches anyway, as they fix more than just this problem. In addition, in Civil 3D 2011 without the Autodesk fix, one of the problems fixed by Owen’s cleanup (a AeccDbNetworkCatalogDef one) is then immediately recreated by Civil 3D.

The upshot is that Civil 3D users should at least apply Autodesk’s fixes; everybody else should use Owen’s.

Using Owen’s fix, it is interesting to see what it reports as being the problem in particular drawings. Here’s what one of my non-Civil 3D problem drawings shows up:

Command: cleanlanguage
Scanning drawing for corrupt objects...
Corrupt object AecDbScheduleDataFormat<2F84> CLEANED
Found 1 corrupt object

Here’s what the Civil 3D 2011 ANZ template shows up when cleaned:

Command: cleanlanguage
Scanning drawing for corrupt objects...
Corrupt object AeccDbNetworkCatalogDef<8B7> ERASED
Corrupt object AeccDbLegendScheduleTableStyle<1619> CLEANED
Corrupt object AeccDbLegendScheduleTableStyle<161B> CLEANED
Corrupt object AeccDbLegendScheduleTableStyle<161A> CLEANED
Corrupt object AeccDbLegendScheduleTableStyle<161F> CLEANED
Found 5 corrupt objects

It looks like every Civil 3D 2011 drawing based on these templates has been going out corrupt in 5 different places. Hopefully, Autodesk will quickly get on to fixing up the Civil 3D template situation, and will incorporate the automated open/save cleanup in future updates to AutoCAD itself and all the other AutoCAD-based verticals.