Update: Looks like this doesn't work in all cases as Kinkless doesn't quite manage to sync completed tasks back from iCal - shame...
Now I've not blogged before about the wonders of Kinkless GTD and the superlative GTD implementation it offers. The wonders of this set of scripts wrung $69.95 from my credit card to purchase OmniOutlinerPro -- heck, it went a long way to convincing me to get a mac...
I'm a big fan of the Plan in kGTD, do in iCal approach Mr Mann advocates on his fine productivity site. To this end, I've discovered a neat counter-hack to the iCal method that's not perhaps as well documented as it could be...
The basic hack is to set up your task calendars into calendar groups in iCal, so that you've got a bunch of meta-contexts you can switch on and off en masse. This is fine and dandy, but wouldn't it be great if you could add a calendar to more than one group?
For example, I have my mac at home and at the office, so I'd like to be able to switch that calendar on and off easily. Too bad, iCal says - only one group for each calendar.
However, after poking around a bit in the kGTD source, I discovered you can do this...

This gives you a calendar which contains a meta-context - effectively it groups different contexts arbitrarily. Note on the screenshot 'calls-day' exists both in the 'cal-office' calendar, and the 'cal-going-home' calendar... This way, I can simply turn on 'cal-going-home' when I leave the office and see all possible things I can do on the way home. Neat, huh?
This is documented in the settings, as you can see, but it was a little opaque to me. Hopefully this'll be useful to someone, anyway...