I agree with Farah Mendlesohn (who resigned from the committee over this choice) that he's a very bad choice for Hugo toastmaster.
My reasons for thinking this differ slightly from hers.
Regardless of Mr. Ross's personality and track record, it is clearly the case that he has a history of scrapping with tabloid journalists, then being quoted out of context.
The problem I see is that while fandom is in the process of cleaning house, inviting him — or anyone with a controversial media profile — to be Hugo toastmaster is like rolling out a welcome mat at the Worldcon front door that says "muck-rakers welcome". There's a lot of muck to be raked, even before we get into Daily Mail photographers stalking cosplayers: just look at the recent SFWA fracas (plural), the Jim Frenkel/harassment scandal at Tor, and so on.
Worldcon should be safe space for fans, and inviting a high profile media personality who has been targeted by the tabloids is going to cause collateral damage, even if nothing happens, simply by making many fans feel less safe.
We're seeing a huge explosion of anxiety on twitter right now. If Ross is toastmaster, I can predict that at least one major Hugo nominee/past winner who was planning to be there won't be present at the ceremony, because Ross has past form for using women with weight issues as the butt of his humour. She says she doesn't feel safe, and I believe her: I wouldn't want to be there in her shoes (and I'm an ancient has-been who hasn't been on the shortlist for a couple of years, now, so I'm unlikely to be in the front row). I don't like seeing my friends mocked, so I probably won't be there either. And this is regardless of whether the mockery would come from the toastmaster, or the tabloid journalists in the back of the audience.
The sad fact is, however well-behaved Mr. Ross is on the day, inviting him into a pulpit that has been misused in the past is sending a really bad signal. (And anyway, what happened to our community's supposed newfound commitment to diversity? Isn't it about time we had a toastmaster who wasn't a white privileged male? Someone like, say, Jane Goldman?)
UPDATE: Stand down:
Jonathan Ross @wossy 12m(Via twitter.)
I have decided to withdraw from hosting the Hugo's @loncon3 in response to some who would rather I weren't there. Have a lovely convention.