I attempted to take a snapshot of a machine while it was running. I noticed that the machine was paused, and then I attempted to resume it. The machine looked like it was going to launch, but the events tab simply indicated that execution of launching the vm failed.
Here's the engine.log at the time:
2012-12-18 22:53:55,838 INFO [org.ovirt.engine.core.vdsbroker.irsbroker.IsValidVDSCommand] (ajp--0.0.0.0-8009-5) START, IsValidVDSCommand(storagePoolId = 2ccd03b1-fd0e-4578-88ce-e5065a9742d7, ignoreFailoverLimit = false, compatabilityVersion = null), log id: 19399f32
2012-12-18 22:53:55,848 INFO [org.ovirt.engine.core.vdsbroker.irsbroker.IsValidVDSCommand] (ajp--0.0.0.0-8009-5) FINISH, IsValidVDSCommand, return: true, log id: 19399f32
2012-12-18 22:53:55,894 INFO [org.ovirt.engine.core.vdsbroker.IsVmDuringInitiatingVDSCommand] (ajp--0.0.0.0-8009-5) START, IsVmDuringInitiatingVDSCommand(vmId = 35d58779-ee24-4926-8612-e053ff48881b), log id: 76d4a2b2
2012-12-18 22:53:55,895 INFO [org.ovirt.engine.core.vdsbroker.IsVmDuringInitiatingVDSCommand] (ajp--0.0.0.0-8009-5) FINISH, IsVmDuringInitiatingVDSCommand, return: false, log id: 76d4a2b2
2012-12-18 22:53:55,935 INFO [org.ovirt.engine.core.bll.RunVmCommand] (pool-3-thread-49) [17252a1c] Lock Acquired to object EngineLock [exclusiveLocks= key: 35d58779-ee24-4926-8612-e053ff48881b value: VM
, sharedLocks= ]
2012-12-18 22:53:55,952 INFO [org.ovirt.engine.core.bll.RunVmCommand] (pool-3-thread-49) [17252a1c] Running command: RunVmCommand internal: false. Entities affected : ID: 35d58779-ee24-4926-8612-e053ff48881b Type: VM
2012-12-18 22:53:55,966 INFO [org.ovirt.engine.core.vdsbroker.ResumeVDSCommand] (pool-3-thread-49) [17252a1c] START, ResumeVDSCommand(vdsId = 0a7046ea-216d-11e2-8fe8-001372eb596b, vmId=35d58779-ee24-4926-8612-e053ff48881b), log id: 607d6dc2
2012-12-18 22:53:55,976 INFO [org.ovirt.engine.core.vdsbroker.vdsbroker.ResumeBrokerVDSCommand] (pool-3-thread-49) [17252a1c] START, ResumeBrokerVDSCommand(vdsId = 0a7046ea-216d-11e2-8fe8-001372eb596b, vmId=35d58779-ee24-4926-8612-e053ff48881b), log id: 66b1872c
2012-12-18 22:53:56,292 INFO [org.ovirt.engine.core.vdsbroker.vdsbroker.ResumeBrokerVDSCommand] (pool-3-thread-49) [17252a1c] FINISH, ResumeBrokerVDSCommand, log id: 66b1872c
2012-12-18 22:53:56,298 INFO [org.ovirt.engine.core.vdsbroker.ResumeVDSCommand] (pool-3-thread-49) [17252a1c] FINISH, ResumeVDSCommand, return: PoweringUp, log id: 607d6dc2
2012-12-18 22:53:56,302 INFO [org.ovirt.engine.core.bll.RunVmCommand] (pool-3-thread-49) [17252a1c] Lock freed to object EngineLock [exclusiveLocks= key: 35d58779-ee24-4926-8612-e053ff48881b value: VM
, sharedLocks= ]
I'm not sure if I was missing something, or if I needed to wait for something to complete, or what. I ended up just stopping the machine and then starting it from cold.