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After adding my first HP ML 110 ESX3i to my brand new HA cluster I got the following error:
The host who gives the error, did not have the ScratchConfig.ConfiguredSwapState option enabled.
This is how you enable this option:
1. In VirtualCenter, select the ESX 3i server with the error.
2. Click the "Configuration" tab page and click "Advanced Settings"
3. Select "ScratchConfig"
4. Set the data store for ScratchConfig.ConfiguredScratchLocation to a valid directory with more than 1GB to hold the swapfile. This location can be local or shared storage.
5. Select the ScratchConfig.ConfiguredSwapState option.
6. Click OK.
7. reboot the 3i server to apply the changes.
Source: http://www.virtualvcp.com/content/view/33/43/
VMware KB: KB1004177
ICT-Freak.nl 
I’ve blogged about this a couple a months ago:
http://www.virtuallifestyle.nl/2008/10/esxi-and-vmware-ha/