motorhaven - 7:38 pm on Mar 25, 2012 (gmt 0)
The gzip/deflate header information is extremely useful, and one I didn't think off. Before any blocking, I've added it to my Apache log format and I'm watching it now. So far it appears that indeed legit browsers (and the legit crawlers I've seen in the past few minutes) send this.
I'll run this for a few days to see what exceptions, if any, there are to this rule for legit users/search crawlers.