Hmm - not sure about this.
I actually love Linux on the PC for the fact that you don't usually have to worry about drivers - you can get a basic setup running very easily. However - when you want something like hibernation or high-resolution graphics then things start to get a bit more awkward - but for a basic internet browsing machine it is very easy to put Linux on a box and get it up and running.
But for a tablet, you have more issues such as battery life etc which I don't see the open software being able to beat the top brands on the market (the ipad).
Plus, whilst you're right about people saying "it doesn't run excel" people will be saying "it doesn't run i-tunes" - so I think that Linux on the tablet will be a very similar experience to non-techys as on the Desktop.
However, what Android and Ubuntu are trying to do is remove the word "Linux" from the brands - which I can see is a smart commercial move. Almost everyone has some sort of Linux device in their house, probably embedded into their router or something - but they don't realise it - this is good for take-up but not so good for the basic principles of free-software...