I had an issue recently where I had installed ESX4 on a number of hosts but had not been allocated IP addresses for them. So I installed them using DHCP.
Later when I had been given the IP addresses I set them on the service console via the vSphere Client, on checking the default gateway it was already set as it had been picked up from the DHCP server. I checked connectivity to each of the hosts while I was oniste but later while working remotely I could not access the service consoles, obviously the default gateway was not correct.
The servers were all IBM System x3650M2 servers with the IBM Virtual Media Key installed so I could access the Integrated Management Module (IMM) and access the console via the Remote Console functionality. The default gatewayhas correctly listed in /etc/vmware/esx.conf but was not listed in /etc/sysconfig/network
The solution was to add the following to /etc/sysconfig/network (I added it as the last line)
GATEWAY=xxx.xxx.xxx.xxx
where xxx.xxx.xxx.xxx is the IP address of the default gateway.
For this change to be picked up networking needed to be restarted by running
/etc/init.d/network restart
This procedure could be used if you ever need to change the default gateway on ESX4. To should be the same for previous versions of ESX.