Richard,
you make an excellent point. I did some research and yes, the Brunton ADC Pro uses Barometric pressure to trigger the weather alarm feature. The Kestrel 4500's pressure trend indicator could then effectively be used to predict the weather. The only drawback would be that there would be no alarm to warn you when the barometric pressure drops in case of an approaching storm. This is a bit surprising to me as it seems like a simple feature that wouldn't be that hard to include in the Kestrel 4500.
Currently my top 3 choices are the Brunton ADC Pro, the Kestrel 4500, and the Suunto X6M. The Brunton has the other products beat as far as price goes but I'm still undecided as to which of the three I want to buy. The only thing I don't like about the Kestrel 4500 is that there is no weather alarm. Both of the other two prodcuts I mentioned have weather alarms.