The kinds of business who get involved in the TAP vary - but mostly, they're large (several thousand users, up to several hundred thousand) organizations who can dedicate resources (time, hardware, even money to pay for external services) to being part of the program.
We have a mix of industry (manufacturing, financial services, service providers, education, IT etc etc) and a blend of geographical coverage (many multinational, some are distinct in geography - eg Japan, central Europe etc).
As for the 'just in case' scenarios - most of our TAP customers will start to implement the pre-release software in test enviornments which aren't necessarily part of the corporate messaging environment (maybe purely a test, or linked only through a separate SMTP namespace), but as confidence grows in the stability of the software they will put it into production in the live system.
At Microsoft, we run a separate 'dogfood' forest for Exchange, so the Exchange team can be testing new versions without affecting the rest of the corporate messaging environment. As we get closer to the release, the MS IT team will deploy the pre-release code into the main corporate forest environment. Only when both Dogfood and MSIT have deployed the new version (and are happy with it), will it be given out to the TAP customers and given the green light for production use.
thanks!
Ewan