I found today a post from a friend (Nuno) asking how to solve this issue, converting an .stl to solid .ipt.
Then I remembered that I found something in the past regarding this conversion... Yeap! it was in Autodesk Labs
So let me explain with couple of easy steps how this works...
1. First visit the following link to download the file and install.
Two quick things/tips when you are about to install the addin I believe is good idea to quite Inventor first.
Plus there are some sample files that you can download and convert these to solid just for validation of the tool
2. After installing open your .stl file as normal
(we are not kids anymore we know how to open a file ;P)
Then right click as shown in the picture and select
"Convert to Base Feature"
3. The pop up window gives you some options... stick with the solid and you don't have to delete the original if you don't want to...
4.Voila! According to the pic below it seems that our model is converted to solid...
At least it looks solid to me :P
Be aware that this conversion it might take some time considering the complexity of the mesh.
I hope to find this usefull as it seems a very rare situation to meet in your ordinary work flow.
However, can be very frustrated by wasting time for something so easy...
Good luck and Have fun...