Not defending either machine, just straightening your facts.
The Acer is more extensible, with user-replaceable battery, savvy-user-replaceable hard drive and RAM (though the RAM is a total pain to get to, not just a door on the back, everything has to come out to get at it) and the possibility for a lot more storage as laptop hard drives get bigger. If it gets stolen, and your data on it is backed up and protected from malcontents, it's $300 and not a huge deal in the grand scheme of things.
The Macbook is much thinner, a little lighter, significantly more powerful with it's 64-bit dual core processor, has reportedly better battery life (Apple's reporting has been pretty close to reality lately) and a full-sized keyboard and almost full-sized multi-touch trackpad (compared to the postage stamp Acer calls a trackpad,) slightly larger screen with quite a bit higher resolution, higher resolution webcam, much faster but more limited storage, can be configured with 4gb RAM (must be done at config time, part of the logic board) and for those that care, Apple's customer service and reliability are rated much higher (per the keynote video.)
Between the two? I'm probably going with the Macbook Air. I don't mind the premium for what I think will be better usability. I'm a larger gentleman with larger hands, and I haven't found a netbook keyboard and/or trackpad yet I want to use for more than a few minutes at a time... too small, slash key and period/comma keys are half size, etc. While the machines are similar in size, the MBAir has it beaten quite soundly in specs. It's really easy to say "haha, Jobs-o made a netbook after putting them down..." but he really didn't. This is no netbook. I had a netbook for two months, gave it to my mother, and I'll never own one again.