The awesome spell for disc is Penance. If it was me, as disc, I'd do what ever I could to get Penance as soon as possible.
If I was going to walk down the shadow tree for anything other than spirit tap for leveling, I'd just go all shadow. Spirit tap while duoing with a mage would probably only benefit you, if you are using holy nova (unglyphed) to contribute to aoe damage. Other than that, you don't have much chance against the mage to get the killing blow. You could also get
a trinket that returns mana when using holy nova, and just get comfortable with the idea that healing priests need to carry water.
While a frost mage can crowd control mobs and aoe grind, a frost mage with pw:shield is even better.
Going disc will work, but remember that going shadow in your duo would also work. You can still PW:Shield as shadow. You simply won't have as good a shields and won't be able to throw shields on multiple people quickly. You won't have your focus as healing. That is something I would want if I had plans to heal as an 80 (which for you wont be too far off).
Edited, Jun 11th 2009 8:20am by dadanox