What happened
We found an End Portal, activated it. We went through and found ourselves standing on an Obsidian square in our Nether (we could see our Nether builds). The dragon had spawned and was attacking us as you would expect. Sadly, I couldn't test if the Nether portal would take us out as we died.

What we expected to happen
We'd go to the End!

Other research
Going back through a Nether portal put us into the Nether without the Dragon.

Mitigating factors
Our map is old - starting in Alpha 1.2.1_01. It has been hosted in a number of different places.