Yeah... I'm wary of "lifetime" licenses as well. Even if the original intent of the license was free upgrades for the customer's lifetime (ie version 4 to 5, version 5 to 6, etc.), I've had a number of programs with "lifetime" licenses recieve no updates for years and then an upgrade for one comes out and the licensing model is changed and the developer wants to charge for the upgrade. Often they'll respond to queries like Curt's above with "Yes, the original intent was to offer free lifetime upgrades but economic realities...". I sympathize with this predicament and have only once written and demanded that the license be honoured (and it was - Collectorz.com software, which, ironically, I haven't used in years). Mostly, though, I either pay the upgrade fee, stick with the last version for which I have a license, or walk away. Increasingly, I walk away.
Just to note, though, that I have a number of lifetime-licenses for programs that have been honoured for years.
I've come to the conclusion that, with the exception of larger companies with very popular programs, lifetime licensing is not sustainable and I take offers of the same with a grain of salt. Overall, my advice would be to write to the developer/sales arm of the company and ask questions about how "lifetime" is defined for licensing purposes and what plans (if any) are in place with respect to grandfathering lifetime license holders in the future should the model prove unsustainable.