I really believe disciple is the best levelling spec for a solo-priest.
Your gameplay won't be so much fun, but you'll be efficent and "fast".
Basically, you're going to shield yourself and wand mobs to death (with the occasional SW:P).
Here's the spec I would use:
http://www.wowprovider.com/?talent=1121 ... 1s5c23001i
Get wands_spec/shield/stamina asap, then Spirit Tap and Meditation.
Main gear focus, naturally, is to keep your wand upgraded, whenever you can.
At level 41/45 you could respecc to full-shadow, but you will be drinking alot...
EDIT:
For real, spec in shadow before you get Shadow_Weaving/Darkness/Shadow_Form is a REALLY bad idea.
Not to mention you will be constantly drinking...after every pull.
Levelling as Disciple/Shield/Wand has no downtime at all...
Also, do not underestimate Improved_Inner_Fire....while soloing you will be hit alot from melee (and casters too) mobs.
Mental_Agility will be good, if you use SW:P on every mob.
Mental_Strenght is good as well....more mana won't hurt...you'll last longer before drinking.
Divine_Spirit your choise...but some more spirit never hurts.