You could have iCal run an applescript that closed all of your windows but one and set that one to the desired perspective. I think if you rummage around in my postings you'll find a script that does just that...ah,
here it is.
One issue with that script that is dodged by having iCal just open another window (without trying to close all the others) is what happens if you happen to have the preferences window as the frontmost window. The script as currently coded just tries to recycle the frontmost window, and the preferences window churlishly refuses to adopt a new perspective :-)