OK, just spent the last couple of days experimenting with my friends ATI Radeon HD 2400 Pro and Im happy to say it works very, very well. I have been able to get vga passthrough working with this with zero issues, no BSODs, no crashes, no hanging, pleasantly solid and reliable. Direct3d acceleration, DirectDraw, movie playback, youtube hd via flash (with and without hardware acceleration enabled) all works first time.
On another note, I thought I would share my reasoning behind looking for vga passthrough, as it will certainly not be the same as anyone elses, I suspect 
My experiment was to see if I could virtualize a windows pc for the sole purpose of running retro oldskool emulators (such as zsnes, kega fusion, MAME etc..) at native sub-vga resolutions out to an old CRT television via a VGA-to-RGB scart cable. To achieve this on a normal physical pc is complex enough, as it requires very accurate display timing software configuration via the use of an application called PowerStrip. To have all of my existing PowerStrip interlaced and sub-vga progressive resolutions working within the vm via vga passthrough, im seriously surprised and amazed it works, given how sensitive the video output is to very small frequency variations.
I have just received my Radeon HD 3450 today (£12 via eBay!) so I will test that next and let you know what my findings were on that, but for the meantime I think the HD 2400 Pro is a no-brainer for anyone wishing to experiment with vga passthrough as its such a cheap card.
For reference, my config is as follows:
Host
ESXi 4.1u1 + all latest available patches
Asus KCMA-D8 Motherboard
2x AMD Opteron 4180 cpus
32GB DDR3 RAM
VM
Windows 7 x64 (fully patched)
2 vCPUs
2GB RAM
VMDP Devices:
1. ATI Radeon HD 2400 Pro PCIe 16x (works great)
2. Creative Audigy 2 ZS PCI (analog audio output is shrill / crackly, but digital SPDIF out is fine)
3. NetMos / MosChip RS232 (1x DB25 2x DB9) PCI Card (works great)
4. VIA 4-Port USB 2.0 PCI Card
Video Driver Catalyst 11.6 video drivers (as of this posting, the latest available).
Regards,
BAM.
Update: 21/07/2011
Just an update to say ive tested the ATI Radeon HD 3450 and it works every bit as well as the 2400 Pro card. Looks like ATI is the choice to make when it comes to vga passthrough. I also added a VIA 4-port USB PCI card - this is detected as three individual PCI devices, 2x usb 1.1 and 1x usb 2.0. Unless all three are passed through, the vm fails to boot. Only the usb 1.1 ports work in the vm (tested ok with a usb mouse and usb keyboard attached), the driver fails to start for the usb 2.0 controller with an Error Code 10 in Device Manager.