Well firstly it's not really £100bn, and that's just one part of the plan.
The £100bn comes from us, the taxpayers, giving a service whereby banks can pay a small fee so that when somebody defaults on a loan (be that company or person, house/car/whatever is covered under the scheme) the taxpayer will agree to absorb a certain amount of the losses,
Another part of the scheme is that banks can declare and kind of isolate the 'toxic debts' (although what determines a toxic debt or not I don't know), I'm guessing they could then get some 'insurance' on this funded by the taxpayer, or it could be used to make the Basel II requirements more favourable to extra lending,
The RBS thing is the government looking like they'll be offering to change the 'Preference shares' (that give out 12% per year 'interest') for more normal shares, this is cheaper for the bank hence why RBS might do it, LloydsTSB/HBOS don't look like they're going to to it.
So 3 schemes, not bad ones, they'll give the banks a little bit more security just as the focus seems to be coming back onto 'how well capitalised are these banks?'. But ultimately it won't stave off the recession, or bring lending back to anywhere near previous levels, but it might take a bit of the edge off.
Problem is they can keep doing things like this and other measures, but we're up a certain creek without a paddle, these measures *may* make the recession less harsh but they are also likely to extend the recession by bringing national debt up, and having banks that want to re-privatise themselves so more likely to spend any free money doing that than to lend it out...