Unlike the iPhone and Windows Phone 7, Android provides a way to install applications that are not in the Android Market. The capability is not enabled by default, so users have to explicitly go into the Android settings to enable the ability to install applications from other sources. We know that carriers or handset manufactuers can remove this capability, as AT&T did with the Motorola Backflip, and I will not be surprised if other carriers remove this feature.
The Android Community blog has a post about an application that allows users to install applications from e-mail attachments that are apk files. Apks are application packages for installing Android apps. I think that one thing we have learned over the years is that e-mail is a popular target for social engineering. Even today too many users get viruses on their PCs by carelessly clicking on e-mail attachments. Providing a way for this type of social engineering to occur on mobile phones is not a good idea. My recommendation to users is to not install this functionality on your phone.