folks buying more than one battery should consider that the Pre's battery contacts are not very good, and repeated battery swapping will probably lead to eventual problems with the phone randomly turning off when bumped/opened/closed/shaken/looked at funny, etc.
also, any battery for sale that's the same physical dimensions as the stock battery is going to have about the same capacity as the stock battery, regardless of marketing claims to the contrary. any cell which is capable of working with the pre's charging circuitry has to be of the same chemistry as the stock battery, and thus will have the same energy density.
I once bought an "extended" battery (for a different phone) and pulled it apart, and found that they had just stuck a "1600 mAH" sticker right over the manufacturer's original "1350 mAH" label.
