I would check your statement that removing the battery voids the MBP warranty, but it would certainly prevent any sort of "soft Landing" in the event of power loss.
As far as I know, there is no lighter weight battery available, and since a lighter weight battery would have proportionately less capacity while market demand is for ever more capacity it is unlikely anyone is going to tool up to make one.
You have already rejected the MacBook Air so that leaves you with a Netbook PC (yuck!) or an iPad. I have recently started carryng a 64GB iPad 2 3G instead of my old MacBook Air and I have never missed the MacBook Air. While I don't use the iPad for heavy data production for occasional or light data production or data use and consumption, I find it ideal. I find it is a great presentation tool and does quite well driving a projector with Keynote presentations, using it as a prompter on the lectern, or for showing information to one or two others. I often find myself using it for email in preference to my 27" quad core iMac. But that is just me.
