With Cata just over a month away it may be worth waiting until its release before you come back. It takes a while to level to 80 and then gear up from dungeons etc to be raid ready and as already mentioned most of our progression teams are slowing down ready for the release of Cata.
Also - by levelling in Cata you will be joined by lots and lots of other people levelling new races and characters so there will be more correct level players around to help with group quests and such. Although, if you join PCG, there is a large base of levelling characters anyway, one of the advantages of being such a large guild.
Cata is also going to bring a lot of changes and it may be easier to simply get used to that than trying to deal with the changes since you last played now which will all be changed again in a months time.
There would be no major disadvantage to being 'late' reaching level cap and getting geared up, especially if you are thinking of joining PCG, we run many raids from beginners to 'hardcore' progression and there are no set groups for these, new players join them all of the time.
Hope that helps