Of the Ultrabooks I've seen so far, the ASUS UX21 and UX31 are the most impressive. They have 6Gbps SATA SSDs (vs 3Gbps in the Air), the same processor choices, and sport USB 3.0 ports (currently a bit more useful than Thunderbolt). The 13" 256GB i7 is $1449, vs $1699 for the 13" Air. Unfortunately, the 11" isn't available with a 256GB SSD, but the 128GB SSD version has a similar price advantage. Getting B&O to build the sound system is another coup, since that company is known for overpriced hardware (they tried to sell a $1200 basic GSM phone a few years back if I recall).
Undoubtedly ASUS (and Acer, who released a $899 13" Ultrabook with an SSD/HDD combo) are undercutting Apple in order to attract business. They can easily see from HP's TouchPad what happens to companies who attempt to match Apple's pricing.
That said, I don't see the Ultrabooks stealing Mac sales very much. They might stem the tide of high-end notebook buyers switching to Macs (i.e. the 5-10% of Air buyers who purchased Airs primarily to run Windows). However, it's a positive development as far as I'm concerned. I'd like an Ultrabook to be my next work PC (we're a Windows shop). It's about time ultraportables become mainstream.