I don't know that much about flying into Kalimantan, flights are sparse and are usually from Indonesia. There may now (my info is about 5 years old) be a small airline between Kalimantan and Sabah/Sarawak.

Between and Sabah and Sarawak you fly Malaysian Air. To get to Tabin, you have to fly from Kota Kinabalu to Lahad Datu. You'll find that your air connections may determine your itinerary. It will likely mean that you lose some time overnighting in KK. Look at the Malaysian Air website, and you can see the flights. We had to overnight in KK before flying to Lahad Datu, then had to overnight again after flying back from Lahad Datu before flying to Kuching. So take a look at the flights, you may find it isn't possible to visit all the places you want in your allotted time.

I'm guessing the object of your trip is to see orangutans. If so, take a look that this (very long) thread from Thorntree.
www.lonelyplanet.com/thorntree/thread.jspa?threadID=307278&messageID=18728971#18728971

It has superb info which has been updated regularly.

From Singapore to Balikpapan there are flights on Silk Air & Singapore Airlines I believe. Air Asia does flights from Kuala Lumpur to Balikpapan.

this is a question for Kathie: Your LP reference has apparently been deleted from Thorntree secondary to their cleaning up their site recently. Do you have a copy of this thread you could pass on to us?
Also wondering when you made your trip. Can't seem to find dates in body of TR.
Thank you.

We were in Borneo in 2005. A long time ago now. It was a remarkable trip.

I'm sorry to see the Thorntree message is not accessible right now. It was writen by a very knowledgeable poster who has lot of experience in Borneo. I do not have a copy, unfortunately.

He contends that the place in Borneo where you are most likely to see oragnutans is the Danum Valley. He also talked about the difficulties accessing Kalimantin.

If I can be of further assistance, let me know.

Generally speaking it's more complicated to travel across Indonesia than across Malaysia. If I were you I'd limit myself to Malaysia, also given your limited time.

You can then base yourself in Kuching (to visit Kuching and Bako) and Sandakan (for Sepilok, Labuk Bay, Kinabatangan river area). From Kuching to Sandakan it's two flights (Kuching-KK and KK-Sandakan). If you want to visit Tabin, take a bus to Lahad Datu.

To book the flights you can use Airasia: http://www.airasia.com/ . Airasia is cheaper than Malaysian Airlines (it's a discount airline).

FYI, the Tabin reserve is very expensive. But if you want to see wildlife, there is a much better place, which is also much more affordable. This is the Kinabatangan river area:
http://www.molon.de/galleries/Malaysia/Sabah/Kinabatangan/

This is the best place where to spot wildlife in Malaysia.

The Thorntree discussion is active again, going back to 2003 beginning.