I encountered the same problem in both the Canopy GUI and on the command line (epkg) on OS X 10.8 with the 32-bit version. This is part of the reply I got from the Enthought support team:
This looks like a familiar bug which should have been fixed.
Could you please update to 1.0.1.1191, and if the problem still occurs, then submit this as a bug report from the Canopy Help menu so we can gather more system information?
Only problem is, I am running 1.0.1.1190, and Help/Software Updates... claims there is no newer version available. I informed support of this, and I will report back as soon as I have more information on or a solution for this problem.
Update
Planning do document the exact error message, I just went back to the command line and invoked epkg, which suddenly did work:
$ enpkg
Enstaller is out of date. Update? ([y]/n) y
enstaller-4.6.1-2.egg
[fetching] 264 KB
[.................................................................]
enstaller-4.6.1-2.egg
[installing] 755 KB
[.................................................................]
Enstaller has been updated. Please re-run your previous command.
I can now successfully install and update packages via enpkg or the Canopy GUI.