This seems to work ;-)
You need to upgrade nginx to version 0.8.3 or newer to get access to this variable. So here are my logs and the values of the variables do match what I'm expected. To test I set the cache to 1 minute.
proxy_cache_valid 200 1m;

xxx - - [31/Aug/2009:03:04:48 -0400] "GET /cal/xxx/851aaaedc0e9debc7d242ca2-xxx/events/ics/attending,unsure,not_replied/ HTTP/1.1" 200 80450 "-" "DAVKit/3.0.6 (653); CalendarStore/3.0.6 (847); iCal/3.0.6 (1273); Mac OS X/10.5.6 (9G55)""[b]MISS[/b]"
xxx - - [31/Aug/2009:03:04:58 -0400] "GET /cal/xxx/851aaaedc0e9debc7d242ca2-xxx/events/ics/attending,unsure,not_replied/ HTTP/1.1" 200 80450 "-" "DAVKit/3.0.6 (653); CalendarStore/3.0.6 (847); iCal/3.0.6 (1273); Mac OS X/10.5.6 (9G55)""[b]HIT[/b]"
xxx - - [31/Aug/2009:03:06:54 -0400] "GET /cal/xxx/851aaaedc0e9debc7d242ca2-xxx/events/ics/attending,unsure,not_replied/ HTTP/1.1" 200 80450 "-" "DAVKit/3.0.6 (653); CalendarStore/3.0.6 (847); iCal/3.0.6 (1273); Mac OS X/10.5.6 (9G55)""[b]EXPIRED[/b]"

Should I use the version 0.8.11 I installed to test it was working, or use the latest 0.7.x version and remove this variable.
Thank you for your time, your comments and your help.