Answer
We believe that the best procedure for doing a phased rollout of Inventory Solution, or any other solution that requires client updates, is:

1. Disable all existing inventory tasks.
2. Allow all clients to update configuration/refresh policies to stop running the inventory tasks.
3. Perform the upgrade to SP3 on the NS server. (This can be done immediately after the solution policies are disabled.)
4. Immediately verify that inventory policies are still disabled (in case the upgrade enabled these policies.)
5. Create a collection of an appropriate number of client computers.
6. Create a new, temporary inventory policy that runs a full inventory to test all types of inventory. (A default full inventory policy can be used if existing collection(s) are removed.)
7. Apply the new collection of a small number of computers to the policy and schedule it to run at a preferred time.
8. When that is proven to be successful,  gradually add more computers to the new collection or add other collections to that policy until all clients have updated the package.
9. Once the rollout is complete, disable the temp inventory policy and enable the new policies. (If a default policy was used, reset the collections applied to that policy.)
10. Depending on the length of time involved in the phased rollout, you may not receive inventory from a large segment of your client computers for some time.