I'd be interested, but I'd be amazed if I could get a plan where voip via phone was cheaper (or even roughly the same) as standard phone calls.
Could anyone provide any info on whether this is feasible?

3G can sometimes take up to 5 minutes just to load the bbc news home page, just depends where you are (not always like those dodgy Iphone ads [the internet...really fast]) - so using voip over it seems a non starter, unless you don't care about high reliability - but In it's defence I guess the tablet isn't exactly a mass market device yet.

Does the quality of a 3G connection depend more on the quality of the chipset used (I know the I Phone is supposed to have a terrible one)?

I know the try it and see maybe the only option, but it could be a costly one! Help and advice appreciated.

I've done some research and if you do any amount of travelling using VOIP and data connection you will need a second morgage for the bill, data connections and roaming are horrendously expensive.
I was advised by 02 to turn off mail delivery for blackberries if you go outside the UK, so having a standard phone would be a requirement if you do any travelling.