Mike, are you thinking of an aluminum small block or big block?
My experience with all aluminum engines is that they tend to leak more due to greater thermal expansion than steel. Even if everything (heads and blocks etc.) are all the same alloy and heating and cooling at the same expansion rates, there's still more movement and walking around on the gaskets and seals so seals tend to wear out faster and leak more easily. Additionally, some alloy castings are so bad that the metal walls are actually porous and can leak right through - you find yourself looking for a cam cover leak or a rear main seal and then you realize the oil is coming right out the side of the block! This is rare, but nevertheless real.
I've been looking into an aluminum block SBF stroked to 347 as this doesn't present too many fitment problems - should save 50lbs over the steel block. What concerns me is what appears to be a lack of additional ribbing or structural reinforcement over the steel block - aluminum isn't as strong by volume and so I can't imagine how a block of the same casting dimensions could have equivalent longevity. Something to think about at least.