As stated in a forum post by Anton Gostev, Veeam Backup & Replication 5.0.2 has still some incompatibilities with the new vSphere 5:
• Incremental replication passes to ESXi 5 target fail (initial pass works normally).
• Backup in direct SAN access mode will fail for VM disks on VMFS5 volumes.
• SureBackup is not functional when ESXi 5 is used to host the virtual lab.
• Multi-OS FLR is not functional when ESXi 5 is used to run the FLR helper virtual appliance.
• vCenter Server 5.0 causes job to fails with the “Inactive datastore” error. Jobs on standalone ESXi 5 hosts do not seem to be affected by the issue, but all jobs configured through the vCenter Server 5.0 connection are.
So, if you’re thinking about upgrading to vSphere 5 and you use Veeam as your backup solution, please wait for Veeam to release a hotfix to address these issues.
They plan to make the hotfix addressing all of the issues above available through support in about 2 weeks. You can follow the forum thread linked above to be updated when the fix will be available.