If you decide to allow your users to upgrade, you need to decide whether that process will be left to the user or only undertaken by IT. While it may seem to add yet another mundane task to the already crippling workload of tech support, it may well make sense to have those responsible for technology oversee the process to nip any problems in the bud and make sure that any settings that need to be changed to meet your policy are changed.
Priced To Move
If you decide against making the update right now, some users are going to be disappointed. This is inevitable. And the $29.99 price for Lion is probably going to make that dissatisfaction even bitterer. They are, after all, being forbidden access to a new toy that costs less than the average business lunch. You know that the costs of moving to a new operating system extend far and away beyond the cost of acquisition of the software.
But your users may not know that. Particularly if they're still pouting about having their request for an upgrade declined by IT. This is an expectation management problem, and once again, it points out the importance of knowing what your game plan is, and sharing it with your users as completely as you can, as soon as you can.