The folks here that argued aluminum would not be used on the iMacs (because of cost, "pro" features, etc. etc.) were shown to be wrong. So I don't think we can put it past Apple to use aluminum in a future MacBook model.
You know I don't think that there's too much value in the whole "pro" designation any more. You don't see an iPod and an iPod Pro. This has been demonstrated in Apple's upselling of the MacBook Pro 15. People that didn't want the smaller 13.3" screen, but didn't need dedicated graphics or large hard drives, splurged on the MBP 15 even though they weren't pros. If I were Apple, I would offer the MBP 15 with Intel GMA x3100 integrated graphics for $1,599, and they would be crushed with orders.
So I don't think it really matters whether or not Apple uses aluminum in its consumer products. In the end, it's the feature set and whether you can afford it or not.