I work mainly with architects and some naval architecs.
I am on civil 3d 2007 plus iv10
I dont know of anyone else that I work with thats on 2007.
a few are on 2004 and most are still on 2000 or even 14.
So I am not upgrading autocad again. the other software that I get files from ie archicad catia etc are still at least one dwg file format behind, so they might catch up in 1 or 2 year's time.

what is the norm in the usa for amount of design offices that keep s/w current. for a developing s/w like iv it still might make sense, but having been on subcription from iv6 to iv10, I cant say I got much value for money. (the picture on the box did not even change, how cheap is that)

I found the most compatible is still ACAD R14 files. IV subcription is almost a joke. Cost too much and every new version won't work until the last SP then new version come and repeat all the problems again.
Many shop either don't have cad or use whatever they can get, old version or cheap knock off.
Its not only on designer. Most CNC punch, mill, lathe, EDM software can't read newer files anyway.

From past experiance, its better to upgrade when needed. Like it clean up bugs and add features that you need. Usually it means skip a few versions. My last employer had problem adding seat for an older version of IV. They're forced to upgrade. This highly encourage illegal copy.

BTW I'm in Canada.

...checkout www.DWGgateway.com if I recall it can open most every release of autocad!

Have that. Came with SolidWorks. Last version keep crashing all the time. This one kind of work but still got problems. AutoCAD is still the only one that can open and edit DWG without any problem.