So, we happend to get an new ESXi 3.5 Server, using an Dell Optiplex G620 for that.
To install ESXi 3.5 U5 onto that Desktop Machine, you NEED to upgrade to Bios A11 and Shutdown CPU Limit ID - and do the Install in SATA Normal Mode (not Compability!). You also need to use the Tipps to install to an IDE Drive (so changing TYPE_IDE to TYPE_ISCSI in the TargetFilter.py) - than it will work flawlessly... (As long as you get the Bios updated, we had the "orignial" A01 Bios and it refused to use an Boot CD for the Update (did load but then always did disable Keyboard - you couldn't enter anything...). We solved the Problem by using an REAL Floppy Disk... Yeah, was hard to get one. That worked! Ah and by the way, the ESXi install did load then, but the Keyboard got Stuck again - we plugged in an USB Keyboard and that one worked, the PS2 didn't wanted to.. well, it always an Adventure setting up ESXi on non-listed Hardware - especially with Dell Bios ;-)...
So - the other Problem is the new and very cool Ubuntu 9.10 - which we did install on the ESXi with latest Software Upgraded. Both Ubuntu 9.10 and ESXi 3.5 U5 (and latest Update by Infrastructure Update Client...). So Problem was: You installed everything, installed VMWare Tools and everything worked - rebooted and your Network was fried... Actually non DNS Lookup did work at all - and everything else neither. DHCP and such worked but it looked like something on the "Hardware" (lol?) / Kernel Level screwed up.
My Co Worker Sebastian came up with one Post, giving an Solution to the Problem:
Unplug the Network from your Ubuntu (by going into the Infrastructure Client, choose Settings of the Machine, choose the NICs and unklick both "Connected" Buttons, press Enter)
sudo rmmod pcnet32
sudo rmmod vmxnet
sudo modprobe vmxnet
Replug the Network
And now it works again!
.....until the next reboot.... T_T'''... >.<'
So.. no final solution here but NOT installing VMWare Tools.
And thats my recommandation after the 5th reinstall of Ubuntu.
Will try again on the next Ubuntu and ESXi Upgrade.