I first thought you might have forgotten to update the Update Manager PowerCLI package but quickly noticed that Get-VMHostPatch is part of the regular vSphere PowerCLI release.
I can confirm this problem on multiple 5.1 vCenter servers managing 5.0 and 5.1 hosts, directly connected to hosts too:
PowerCLI Version
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VMware vSphere PowerCLI 5.1 Release 1 build 793510
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Snapin Versions
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VMWare AutoDeploy PowerCLI Component 5.1 build 768137
VMWare ImageBuilder PowerCLI Component 5.1 build 768137
VMware License PowerCLI Component 5.1 build 669840
VMware VDS vSphere PowerCLI Component 4.1.1 build 000001
VMware vSphere PowerCLI Component 5.1 build 793489
VMware vSphere Update Manager PowerCLI 5.1 build 782803
Seems like this isn't quite a new issue either:
http://communities.vmware.com/thread/336323
So it looks like the only reliable check for patches at the moment would be via Get-EsxCli?