Just wanted to point out that while "Android" does in fact let you install apps from any source, many Android phones (for example virtually all of the ones offered by ATT) do not actually allow installing non-market apps. And as this is something that any carrier or vendor can implement, I wouldn't be surprised to see more of it in the future. (Much like how even though 2.2 supports tethering, so far handset manufacturers/carriers have been disabling that support on their 2.2 rollouts)

People always seem to ignore this.