Here's my perspective on this... take it for what it's worth.
Today, I have a Dell Vostro V131. It has a 65W extended battery. It runs a Generation 2 i5 processor, 8GB of RAM, and 256GB SSD, so in other words, it's comparable to the Surface Pro aside from the fact that Surface Pro has a generation 3 processor and a 48W battery. Of course, this screen is 13" instead of 10" too, so keep that in mind as I continue on...
The Vostro is advertised as having 4-6 hours of battery life with this extended battery. That was definitely true when I ran Windows 7 (averaged about 4.5), but when I upgraded to Windows 8, it nearly doubled to more like 8-9 hours, averaging 8 hours to the 20% mark (I never let my system go lower). Now, I say that, but there's the usual conditions of usage being applied... I dim the display to about 40% (not precisely, but about), I keep Wi-Fi on and use a Bluetooth mouse, but this isn't running full video or anything power hungry... it's just general web surfing and general work. Call it "casual use" with no "sleep" time. That, in my opinion, is absolutely phenomenal battery life. Nothing short of spectacular and amazing for a full-time system of these specs.
Surface Pro obviously has a smaller battery, but it's also BIGGER than the Surface RT battery. My Surface RT generally goes about 9-12 hours of active use (I haven't done any official tracking but 10 hours is a fair average), and goes DAYS on idle/standby use. It's actually improved since the various firmware updates that have released. So, here's what I'm expecting from Surface Pro.
I expect the 3rd Generation i5 processor and graphics chipset to be BETTER than the 2nd Generation processor I have today in terms of power consumption/management. I also expect the smaller screen (even at higher resolution) to consume less energy than the bigger display. I expect Microsoft to also take advantage of new motherboard and other chipset options to eek out some additional improvements in Windows 8. In the end, with a 48W battery, I expect Surface pro to have 5-7 hours of battery life with the same usage as my Vostro.
I anticipate that Microsoft will market it as 4-6 hours, and of course, mileage may vary from user to user. If that's a problem for you, then absolutely, don't buy it. If that sounds good for you, and you like everything else about it... go for it. Same as any other spec comparison. I'm not sure on the "average life" of other ultrabooks, but real world experience as an IT guy tells me that when you approach 8-10 hours for any full-time device, it's amazing and a realistic high is that 6-8 range.
Of course, if you're automatically claiming that it "sucks" without any basis of fact or based on a tweet, you might be an idiot... or at the very least, you need to be more realistic.
I'd love to know that I *can* be on battery all day for work, but the reality is I literally NEVER need it that way. I'm always at my desk, I'm always plugged in, and I go mobile for only brief periods during the day.
PS-- I'm still waiting for the launch to decide if I'm getting one. I desperately want a tablet with Type Cover and an active pen digitizer for OneNote usage... but I feel my system must get 5 hours on battery to be useful to me. I'm actually more concerned with other factors like fan noise and whether or not 128GB is enough storage for me.