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Perfect Forward Secrecy - is it worth it?

Quick survey; how important is perfect forward secrecy to you? I've asked 
three people locally so far and gotten four different answers, so in the 
spirit of spreading divisiveness where'er I go, I'll try and get a few 
more opinions here :-) 

In general, schemes offering PFS require a extra PK-op, and an extra 
round-trip when compared to  non-PFS schemes. This cost is incurred once 
per "session", but can add on the order of seconds to startup times. 
Should key-management schemes where PK is available always provide PFS, 
allow PFS, or not provide PFS? The amount of code needed to implement 
each choice point is similar, if you're using something like BSAFE. 

(defun modexpt (x y n)  "computes (x^y) mod n"
  (cond ((= y 0) 1)  ((= y 1) (mod x n))
	((evenp y) (mod (expt (modexpt x (/ y 2) n) 2) n))
	(t (mod (* x (modexpt x (1- y) n)) n))))