I cleared the market's cache, but that didn't seem to help. (Since I
bought the Direct Purchase version, I'm not installing from the Market
anyway. I can't even find Tasker in the Market.)
I have turned on settings>applications>unknown sources. That's what I
meant when I said that I checked "Allow
installation of non-Market applications" — on my phone, it says both
"Unknown Sources" and "Allow
installation of non-Market applications" next to that checkbox.
I'm unable to install another non-Market app now, too, so I'm starting
to think it's a problem with something on my phone — possibly
Cyanogenmod, since I didn't have any problems like this before
installing it. If anyone has any more suggestions, please let me know.