One of the great advantages and potential minefield of Android, is the simple fact that an application does not need to be in the Android Market as Spotify have clearly illustrated
http://www.spotify.com/en/mobile/overview/install-for-android/
Clearly this is aimed at the Finish owners of Android handsets where there is no Android Market.
The downsides of this ad hoc distribution is that an application could potentially be malicious and just alter the users dialler to re route calls through a premium number, (similar things happened back in the days of dialup with peoples machines getting compromised by malware)
However the pros for the consumer and third party developer are much like the internet and that seems to manage quite well despite the threat off malicious software getting installed on an end users device