Whilst working on a customers site I discovered an issue trying to customize a Windows 2008 Datacenter x64 template, whilst trying to discover if this was an issue with just this OS I have found the following.
The below section is taken from vCenter 4 administration guide
http://www.vmware.com/pdf/vsphere4/r40/vsp_40_admin_guide.pdf (Page 176)
This is the same section from vCenter 4 Update 1 administration guide
http://www.vmware.com/pdf/vsphere4/r40_u1/vsp_40_u1_admin_guide.pdf (Page 176)
Note that Windows 2008 support has been removed from the update 1 administration guide.
I would be interested to know if anyone has been able to customize Windows 2008 VM’s using update 1 and if you have what version of 2008 and was your vCenter a fresh install of update 1 or an upgrade.
@vmwareKB on twitter is currently looking into this for me at the moment, will update you with more findings.
Update
A number of people in the comments and on twitter have told me they have been able to customise 2008 standard and enterprise with update 1, I have reinstalled the VMware tools in my Datacenter template and have now successfully been able to customise my VM. So this looks like a section has just been missed from the documentation. Will let you know when I get an official update from VMware on 2008 customisations.
Virtualised Reality 
I’ve recently deployed 4 Windows 2008 Enterprise 64bit servers from template using vCenter 4.0 U1. No issues in deployment or customization.
Yes I managed to customize Windows 2008 (32bit) Standard VM on Update 1 without any issue.
I’ve recently built a 2008 R2 Template and deployed VMs with a customization specification from it.
Before U1, I would get an error that i can’t use guest customization for 2008 R2.
I had to create a new customization specification though because the VMware sysprep process would run into errors with my working 2008 R1 specification (which have been due to old licensing information, I wonder).