I've written a primitive AppleScript called iCalTextImport to fill this gap. The script works for me, and may be of some use to others. It uses the Satimage osax and Prog_Bar 0.2, both of which are freeware and which are included in the package (acknowledgement is given in the ReadMe). Feedback welcome!
[robg adds: I tried to add a link for Prog_Bar, which is from Spork Software, but their website seems to have vanished and the program hasn't been updated in about a year. I haven't tested this script...]

