Good comment about the isolation transformer - that definately adds a safety aspect, and you can then bond the ground and the neutral coming out of the transformer.
As far as I understand, with grid power, the voltage difference is as follows:
Live to Neutral --> 120V
Live to Ground --> 120V
Neutral to Ground --> 0V
If you have 240V power, then it is:
Live1 to Neutral --> 120V
Live2 to Neutral --> 120V
Live1 to Ground --> 120V
Live2 to Ground --> 120V
Live1 to Live2 --> 240V
Neutral to Ground --> 0V
With a configuration like that, since there is no voltage between ground and neutral, joining the two does not cause any problems.
The problem with inverters is that there is voltage between the neutral and ground, so joining them causes a huge amount of current to flow from neutral to ground, blowing something.
So that is the basic theory, but there are some interesting things I have noticed:
I just checked my cheep 400W MSW 'Chinese' inverters, and they don't even have a ground! There are no wires connected to the ground on the sockets, and no voltage between live and ground or neutral and ground! So in this case, they simply did not bother with a ground. Interesting.
This means that you could probably just use the live and neutral from your inverter, and leave the house ground connected to the circuit. The only problem with this is if the negative terminal on your battery bank is grounded, you don't exactly want the high voltage neutral on your inverter directly connected to the batteries!! Shouldn't be a problem though since you say that your battery bank is not grounded. I don't see why there is any point grounding your battery bank. Anyone?
Basically, the safest thing to do is to use an isoltation transformer, and bond the neutral output of the transformer to the house ground. So you can use one of those 3-way light switches, without worrying about the ground and all that, if you use an isolation transformer.