I would recommend against starting this now. Most DPS are in AoE mode for dungeons making it hard for experienced tanks to hold aggro in 5-mans. My advice is to honestly wait until cata unless you actually have a premade 5 man you can take with you that will help you to learn.
There are a couple of specs I am using currently for prot, with the point allocation changing based on whether it is for threat or for survival. For threat I use a spec that does not include victory rush, safeguard, or vigilance. Thunderstruck and Blood and Thunder are lack-luster unless you have packs of mobs of 10+, so I'm not convinced of their viability currently.