Will small businesses and mid-size companies begin to question the wisdom of paying a full-time email maintenance staff to keep their email lifeline flowing, when they can avail themselves of "free" (easy-to-use email, calendaring, Web page creation, and hosting, etc.) from Google? You bet!
Will large companies move their employees off of Microsoft Exchange or Lotus Notes to Google Apps and Gmail? Probably not.
But will large companies allow employees to use Gmail as a back-up email alternative for situations in which their corporate email is compromised by virus attacks or inaccessible? Absolutely!
Maybe true, but there is a third way between "big companies won’t migrate off exchange" and "small companies will use this as a cheap alternative to enterprise software." It’s "individual professionals will adopt cheap or free tools to realize productivity gains, and to heck with what the IT guys say the corporate standard is."
I was working at a company a while back that tried to migrate everybody off AIM and onto a secure IM client. That initiative went nowhere; individual users were so far ahead of the corporate IT team that it was easier for them to just install the free AIM client and use that. When a few key adopters installed AIM and used it, more people installed it because their co-workers were using it, and it was only a matter of time before AIM became an unassailable folk standard within the company.