What I have: Ipaq 6315, USB Bluetooth adapter using BlueSoleil ver. 1.6.1.4 and T-Mobile with International Roaming (Unlimited Internet Data Plan in the U.S.)
I was very frustrated, I couldn't get it to work and 99% of the search results were others asking how to make it work. The solutions/instructins I found were not working. I ended up using bits an pieces from a few different instructions. I am posting this from my laptop using my Ipaq 6315 for internet access connected to T-Mobile on a Bluetooth Dial Up Networking connection.
Here is how I did it.
1. You should have already paired your 6315 and your laptop through BlueSoleil
2. On the IPAQ, Turn on the DUN Service (Dial Up Networking) from the Bluetooth Settings section on the IPAQ.
3. Refresh the connection to the IPAQ in Blue Soleil

4. Click on the DUN Icon in BlueSoleil

5. You will then get a DUN Screen, if something is entered in the Username field, erase this and leave it blank. Do the same with the password field.

6. The string that is in the Dial field is WRONG! Erase it and use *99#.
Bang! It should work unless I forgot a step. If I have, please let me know and I will edit this post.
I hope that I can help others with this info. I know how frustrated I was!