Before dual spec, I don't think you should have much problem healing as Ret up to about 35 as long as the dps are not trying to tank as well. As far as sticking to one spec and filling two roles, there is a thread on the general board of a member tanking as Ret all through the Vanilla instances, so you'd be able to be Tank/DPS all the way up to 58 if that's what you want to do. Again though, for healing, I wouldn't chance it past 35 when you start hitting instances like SM Cath, RFK and ZF, assuming you're trying to heal in Ret gear.
Edit: I was wrong, the thread I was referring to, he tanked till 40 as Ret, and then switched.
Edited, Feb 3rd 2010 3:16pm by EpicUnderwear