I have benefited a lot from your threads thanks Michael, it seems the majority opinion is on complementing Aikido with BJJ, thefore the question now is: would you train in both martial arts right away or concentrate your effort into one until reaching black belt then explore the second?
I endorse Kevnin's response. Black belt is probably not necessary (particularly for BJJ where gradings are a little differen), but a reasonable grounding is a good idea. The trick I think is to time the looking of a second art right so that
1. you aren't soo invested in your current art that you are only looking to discredit or discount the second art.
2. You are invested enough in the current art that you desire to keep training in it rather than dropping it immediately for the cool new toy.
After that you may find like Kevin that you continue to train both or you may find after a few years you wish to concentrate more on one (which I have done).
My sequence was pretty much ideal for me (and maybe only for me). i.e. I was advanced enough in Aikido to not want to let the Aikido go and keep training. I was also advanced enough to see quite quickly how similar the underlying principals are. I then did both for a number of years before deciding to concentrate on BJJ as that was where I have the biggest skill gains to make. I will do this for another few years. Once I get to a higher level in BJJ and find myself with more time on my hands due to lessened family commitments I fully expect to dive back into Aikido, bringing with me the lessons I learned from BJJ.
But that's just me.