UPDATE: I found a workaround. You need to unistall nuget pm and install the new version from the gallery.
I have the same problem - the installer says that the current version signature differs from the signature of v 1.5 Here is my log:
2VSIXInstaller.Common.VerifyMatchingExtensionSignatures(IInstalledExtension installedExtension, IInstallableExtension updateExtension) w VSIXInstaller.InstallProgressPage.BeginInstallVSIX(SupportedVSSKU targetAppID) 2011-12-22 09:32:53 - Install Error : VSIXInstaller.SignatureMismatchException: The signature on the update version of 'NuGet Package Manager' does not match the signature on the installed version. Therefore, Extension Manager cannot install the update. w VSIXInstaller.Common.VerifyMatchingExtensionSignatures(IInstalledExtension installedExtension, IInstallableExtension updateExtension) w VSIXInstaller.InstallProgressPage.BeginInstallVSIX(SupportedVSSKU targetAppID)
I have the same signature mismatch issue but only for 2010 Ultimate the signature works for 2010 Express (didn't realize it was on my box). This may help you with the problem of how you have the install signed.
VSIXInstaller.SignatureMismatchException: The signature on the update version of 'NuGet Package Manager' does not match the signature on the installed version. Therefore, Extension Manager cannot install the update. at VSIXInstaller.Common.VerifyMatchingExtensionSignatures(IInstalledExtension installedExtension, IInstallableExtension updateExtension) at VSIXInstaller.InstallProgressPage.BeginInstallVSIX(SupportedVSSKU targetAppID) 21.12.2011 10:46:43 - Install Error : VSIXInstaller.SignatureMismatchException: The signature on the update version of 'NuGet Package Manager' does not match the signature on the installed version. Therefore, Extension Manager cannot install the update. at VSIXInstaller.Common.VerifyMatchingExtensionSignatures(IInstalledExtension installedExtension, IInstallableExtension updateExtension) at VSIXInstaller.InstallProgressPage.BeginInstallVSIX(SupportedVSSKU targetAppID)