Apple has a B2B program where you can make an app available only to phones registered to a particular organisation. It has not been rolled out worldwide, like the google program appears to be:
http://www.apple.com/business/vpp/
On the downside of googles program, it appears to rely on a company using google apps (basically the corporate version of Gmail, which runs under your own domain). I understand not many companies use this - its definitely not as popular as say - exchange.
It seems that google is slowly migrating google apps platform to not just being reliant on using all googles apps/services. IE: Postini users will be migrated next year to Google Apps (but not require to be using gmail).
Gmail is much more popular than exchange. The types of businesses who are chained to exchange are not the type to be deploying custom android apps in house.
On the downside of googles program, it appears to rely on a company using google apps (basically the corporate version of Gmail, which runs under your own domain). I understand not many companies use this - its definitely not as popular as say - exchange.