I'm running Linux 2.6.x inside a VM on VMWare ESXi 4, and I am wondering where I can get any decent entropy for cryptographic operations. The hardware ESXi is sitting on doesn't do passthrough, so does anyone have any ideas?

What kind of passthrough do you expect? Linux doesn't use any hardware dongles to generate entropy in /dev/random (there are dongles available though that are capable to generate high secure one time pads). Therefore this hasn't to do anything with VMWare but with the entropy pool your virtual machine may collect.

This pool is filled by timing events, interrupt events, input events and disk I/O. Verify this here in the Linux source code. If you now live in VMWare or not, just make sure one of this collectors is able to gather some events in order to receive more entropy.

Linux virtio support includes the virtio-rnd device which provides an emulated pci hardware entropy device. Guest OSs can stir this into their kernel pot using rngd.

Of course, what entropy gets sent through virtio-rnd is up to the virtualisation host. I think KVM lets you pass a stream from a char device, such as a HW RNG connected to the host through (support only recently added).

AFAIK, vmware has no equivalent. You might look at Entropy Broker (available from vanheusden.com) to distribute some randomness to your guest.