I've wrestled with this issue a lot in my organization and you've given me some things to think about I hadn't already. Thanks.
My approach may not work for some because we each have our own particular situational imperatives. The policy I've implemented is relatively simple:
1. End users (customers) are supplied PC's that restrict admin privileges by default.
2. A customer whose job responsibilities require travel (in our case all travel is international and to very remote locations) will be granted limited admin rights so that in the event software necessary for the performance of job functions in the field must be installed the customer will be able to do so.
3. A customer whose job responsibilities do not require travel does not require admin rights. Any and all installations will be performed by IT, and then only once all the appropriate QA/QC activities have been completed. We guarantee rapid turnaround time for this via published SLA's.
4. We protect our laptops with the same set of controls we establish at the network perimeter. Theoretically the laptop security is sufficiently strong such that it can't be defeated no matter what the customer does. In the event laptop security is breached, the network perimeter controls are sufficient to avoid system infiltration. (In the last 12 months we've undergone hundreds of intrusion attempts and have been 100% successful in repelling the attacks, so we don't think we're kidding ourselves.) Under no circumstances is anyone outside IT granted any level of network admin rights.
5. In the event any customer performs an operation as a consequence of having local admin rights that affects the laptop in any way, IT will not attempt to diagnose or repair the problem...we will simply reimage the machine. If any data loss is experienced as a result, IT is not responsible since we require all files be network resident. Customers violate this policy at their own risk.
This entire policy is published, and we require all network users to sign a technology use and security awareness policy...when they join the organization and every year on their service anniversary...acknowledging they understand and will adhere to the policy, accepting sanctions for non-compliance up through and including termination.
I don't know if this is helpful, and I'd actually appreciate a little feedback on what folks think about how we do this.