okay, assuming that your now partners knows you are seeing someone else as well as them and assuming they know who it is and assuming that you aren't cheating... I would think that they would be a shoe in for being okay with their friendship continuing after the adjustment time of starting a new relationship occurs. Asking what is okay to talk about and what isn't would be appropriate. Asking what they want and fessing up to not knowing what the hell to do would be a great way of letting them know that you are needing their input and will do as they wish.... very respectful move I think. Respect, communication and honesty are good foundations to poly... at least in my experience.
If my assumptions are off and you have been cheating or not telling you partner who you are also dating then I would have to seriously consider where you will go if she blows up. Find a place to stay that is going to give her some room and space to think (just in case) and then just say it. "I have another lover." I'm sure there is a lot of questions she will have and you can answer calmly knowing that you need to stay calm and allow her this time to get through all her emotions. You will have to have a lot of patience and give her all the time she needs to get through the huge breech of trust that occurs when trust is broken by cheating.
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