It's a really really bad idea.
The banking sector's profits in 2006 were just under $800bn. The organisers of the Robin Hood tax say that they could raise $400bn annually. So we have to imagine the global banking sector would happily offer up more than half of all profits, and NOT pass additional costs onto their customers. They won't, and this tax relies on every bank in every country agreeing to it.
Added to this is the fact that a decent proportion of transactions have a profit margin around the 0.05% figure the RHT is proposing to hit. The result would be that these transactions would no longer be viable, and would not take place, thus cutting the amount of money this tax could actually raise. Interestingly, this was the original idea behind the Tobin tax in the 70s, to dampen down the market in speculative trading by making it uneconomical to do.
There is no actual discussion on how the 'high-value' trades to be taxed would be split off from the smaller-value transactions that many of us make on a regular basis. Not good news for people who are worried about pensions (stocks and shares) and buying foreign currency.
The final nails in the coffin for me, though, aren't strictly economic ones. When this tax is being proposed by celebrities who I am sure have all benefited from off-shoring of assets, and is to be used on increased public spending rather than cutting the defecit and getting us out of recession earlier, it's a bit of a non-starter...
Oh, and didn't Robin Hood fight AGAINST taxation?