Lantau has a lot of riding on it, so if you're stuck anywhere with a bike, that's one of the better places. Note though that a lot of that riding will not be on legal trails.
If you need to get off Lantau, your options really depend on where you live:
The DB ferry takes bikes at no extra cost and is frequent. Living in DB doesn't seem to stop riders coming over to HK on the weekends.
If you live in Mui Wo, the slow ferries allow bikes, but the timing is not the most convenient for linking up to other riding areas. There are several dedicated bikers out in the Pui O area that you could hook up with though.
If you live near Tung Chung, you can catch the airport express by riding up to the airport terminal. You can also arrange a man-with-a-van at a cheaper cost than a taxi ride (and you can share the ride with a bunch of other bikers).
Presumably you already know where to get a bike, but I'll mention a couple of places that may be local to you anyway. Friendly Bike in Mui Wo is very good. There is also a store near Tung Chung run by a friendly guy called Sunny (hard to find). Don't use the store in Wan Chai.
Get in touch with the guy who runs a bike guiding service on Lantau if you want more info: Cross Country Hong Kong