Bitcoin won't be useful until at least 100 million people use it. We'd really like 1 billion people to use it. But the maximum number of users possible is 21 million, because only 21 million people can have bitcoins, even if each has only one.
Bitcoins are divisible to the one hundred millionth place. So the code can support up to 2100 trillion users.
It's very discouraging to start up your bitcoin client every day for weeks, and keep it running all the time, and still have no bitcoins. This will discourage early adapters.
Early adapters are a bit more robust than that, I think. But there's a thread on this topic in the bitcoin forum, so perhaps the button will be removed in future versions. Ewallets like MyBitCoin do not have this problem to begin with, and it appears that the Google version does not have this feature either.
Bitcoins are divisible to the one hundred millionth place.
Actually, it's better than that. The software can be updated in the future to make them more divisible, even just by shifting the decimal places on their internal representations (thus, start using 'bitpennies' or something.)
Though not yet an "official" project, Google has released a Bitcoin client. As you may remember, there were concerns here about what the government/legal reaction to Bitcoin [1] will be, and the significance of certain groups lending their support to it. EFF and SIAI accept Bitcoin donations, which helps, and this action by Google is another big step.
Previous articles: SIAI accepting Bitcoin donations, Discussion on making money with Bitcoin (Clippy warning on the latter)
[1] In short, it's an anonymous P2P crypto-currency with no transaction fees, in which new units are generated by spending computer cycles computing hashes until you find one with specific properties.