This is a dodgy issue, and I'll relate my experience. The basic issue is needing an IMAP email account (rather than a POP account) -- this allows each machine to access email and get a copy of the email rather than the original. This is how corporate networks operate so you can get your email from anywhere, even simultaneously.
Most consumer accounts (AOL, earthlink, etc.) are POP accounts and cannot do this -- they were not designed considering you might want your email at multiple machines. With a POP account, however, you can set a preference in your mail program (entourage, outlook, mac mail, etc.) to get a copy of the email and leave a copy on the server for a set period of time. This way, when you access email from the other computer it will download the same email (provided it's within the time period you specified for the POP server to keep email, e.g., 30 days). The downsides to this approach are mostly (a) if you get a ton of email you might run out of space on your POP server since it's keeping everything for some period of time, and (b) you have to "manage" both your computers' in-boxes (that is, if you delete an email off of computer 1, it's still in computer 2's inbox until you delete it there as well).
The .Mac account is an IMAP email and you can set each computer up for that account and it will work as you desire. But as you noted it's not cheap. Any IMAP solution other than building your own server will be an over-the-internet solution, but there are cheaper IMAP services out there.
For me, .Mac was useful because it provided a solution for sync'g address books and calendars, but not email because we use our DSL email account and it can't be sync'd (so we use the POP solution I mentioned above). When the discounted trial period expires, however, I hope to have found another IMAP service that is cheaper.
