The GMail Drive freeware program, developed by Bjarke Viksoe, allows you to do exactly that. At first glance it seems like a slick idea, but is it smart to use it? Probably only if you use it for storing information that is safe for other people to see (technically "all your data are belong to Google") and only in cases where you can afford to lose that data if Google should decide to delete your accounts. With that in mind the GMail Drive tool seems rather useless.
It seems to me that people who need a legitimate way of making their data available wherever they are could more easily buy a laptop. Even a decent used laptop can be obtained for a few hundred dollars these days. Then again, some people want all the bells and whistles they can get regardless of whether those bells and whistles make any real sense.
From an information security perspective the tool certainly presents a security risk. So do free mail accounts.
The idea of free mail accounts is attractive and could possibly serve a useful business purpose. But again, companies need to keep in mind that storing mail on other companies mail servers (Yahoo, Google, Hotmail, etc.) could lead to information leaks or data loss beyond your control. Even so, I've seen employees of companies using free mail accounts for business reasons.
How does your company address the use of free mail accounts? Do you have a policy in place that prohibits such use for company-related activity? Do you provide your own Web-based email sevice for your mobile users? Do you block traffic to free mail sites? Or have you found that free mail services offer a benefit to your business?