I didn't expect so many people to be hostile to the idea of having some emergency cash on your bike. It's just a $20, if your bike gets stolen, you have much bigger things to worry about. On the other hand, if you just happen to forget your wallet, a $20 stashed somewhere could be very important.
In any case, an extra water bottle can be a nice place to store stuff like money, keys, etc. A cheap water bottle isn't going to attract the attention of 95% of thieves (a pump... I figure there are at least a few thieves who probably thing a mini pump is some extraordinary feat of engineering and therefore worth money) and nobody expects anything but water to be inside of them. Specifically I recall a thread from Commuting where a rider going home at some ungodly hour was stopped and shaken down by the police (for no real reason other than people out at 3 am are a little suspicious by nature). I believe he consented to a search or something... Point is, the cop checked his pockets and his seat bag or pannier very carefully but never checked his water bottle (where he had his keys and other items). Think of all the drugs you could fit in a water bottle!