Sorry to resurrect this a little, but taking that I live on the Rock, I thought it might be useful..
QUOTE
The Isle of Man TT course is opened to the public for one Friday during the TT. Anyone on two wheels. As I understand it, the roads (being closed to general (4-wheeled) traffic), have no speed limit. (There's no speed limit on rural roads on IOM anyway.)
Erm..It's a Sunday, actually - hence the name "Mad Sunday"

That's when the mountain road is usually made one-way, and anyone (two and four wheels) can go for a more-or-less civilised run over the mountain. It's different this year, as the mountain road is one-way for the whole two weeks of TT. Doesn't mean it's Mad Sunday for two weeks, though, and ye olde bill has already announced a permanent presence up there - basically, normal rules of the road but one-way to avoid head-on collisions, which unfortunately have cost many a life in the past.
Although it's correct that some rural roads don't have speed limits, many do, and the number is unfortunately growing thanks to some civil servants here liking to follow the nanny-state approach of the UK. We narrowly avoided getting an all-island speed limit last year...
Other things to keep in mind:
The TT course is pretty nice and smooth, but that's because most of the cash is spent on it, leaving a little less for the other roads, so expect potholes and bumpy sections away from it.
Local drivers can not negotiate roundabouts - it's impossible - they tried. It's mexican stand-off every time - hopeless, and no-one knows why. Must be the air or something..