Lightning,
If you imagine the scenario in which a website 'goes down' (ie is taken off line in a fast directional market to prevent themselves from being taken to the cleaners), then do you really think your stop would be honoured anyway? Or might there be a little bit of slippage In the instance that it's a genuine site outage and you urgently need to close a position, you could always enter it with one of the other spreadbet firms you have an account with for these eventualities, and close both trades out when everything is working okay again. To echo Splitlink's point that has only ever happened to me once in all my years of trading and it was a conventional broker who let me down (site down, not answering phones) and a spreadbet firm that bailed me out by letting me offset the trades I wanted to close!
I think that the broker registers the stops at the exchange so, in a broker's site crash, you should not be affected. My problem, that morning, was with traded options. The exchanges do not accepted trailing stops, though. These are a broker's service to his clients and would be affected. IB warns traders on this point.
Split