If you're flying into Kota Kinabalu then that's pretty much a developed town (in an slightly old-fashioned sort of way), so it's probably not what you'll expect, but other parts of Borneo, especially Mulu and also maybe Sepilok (where the orangutangs are) are probably just like you imagine, except only better in real life. Flying to Mulu from Miri was excellent (it was a small 50 seater plane and the scenery was class), and then Mulu itself is class. The scenery reminded me of sort of prehistoric scenery and the airport is just a runway with one building and the whole attitude of the airport staff was so laidback, so from the moment I arrived I just thought, 'Aye, I'm gonna like it here, like.'
For accomodation in Borneo, there's quite a few hostels in Kota Kinabalu, so it's easy enough to find somewhere to stay there, but I pre-booked somewhere anyway just to make sure. For the rest of Sabah I was on a pre-booked tour so they sorted out all the accomodation, but most places we went to seemed to have spare rooms/beds. The exceptions to this were Turtle Island, which I think you would definitely have to book in advance, and also the overnight place halfway up Mount Kinabalu which again you would have to pre-book. But if you're climbing the mountain (although it's actually a hike not a climb) then I think you have to do it with a guide so you would book your bed as part of the whole deal anyway.
For Sarawak and Brunei, I traveled round with people I'd met in Sabah and in almost every case we just turned up and found somewhere straight away. At the place we stayed at in the Mulu jungle though, we would probably have been better off booking in advance cos they only had space in the dorm, and the cost of the private rooms was only a bit more than the dorm so mebbees if you go there I would recommend booking in advance, but other than that I didn't have any problems.
Also, in Mulu the official tourist board place is definitely the best place to stay as it's right at the entrance to the park and all the treks. There's a smart hotel a couple of miles down the road from the park, and it's a nice hotel but really if you're going to Mulu then it's to see the caves with the bats and to do the treks and stuff, so the official place is definitely much better located.