Brian,
There is a well documented SNMP bug (MIB2) on most (if not all) devices with regards to "Last Boot," so I would not recommend that alerts or reporting be based on this information. The bug reports that device as rebooted whenever the SNMP counters are reset on the device. For alerting, we use ICMP polling. We've tweeked the alert so it takes several failed ICMP responses before an alert is generated, and it works out pretty well. This stat also provides info for availability reporting.
Now, if you have Cisco devices in your environment for networking, you should alos look at configuring IP SLA and evaluate Orion VNQM (it's not just for voice monitoring).
D