Apart from my two lithium ion batteries, I always carry at least two spare sets of AA's, one set pre-loaded into the cassette. The ability to throw in a set of AA's is the main reason I use a grip. AA's can be bought in little, out of the way villages in the back of beyond, BP-511A's cannot.
As you can tell, I've been caught without a spare charged battery once too often.
The 2000 shots on one battery is amazing. When I was testing my 30D after buying it, I only had the one battery and went from fully charged to nothing in about 50! That was using AI Servo and focusing/metering a lot but taking few actual shots.
Richard