I reviewed Thinkpad X1 Carbon, Samsung Series 9 and both Zenbooks recently (tech journo here) at our test lab.
Samsung has the best battery-per-weight ratio, but it has the worst screen out of the three. Still better than average, except its gamut which is typical = bad. The screen is the brightest out of three and 2nd best in contrast after Zenbook. The mouse has some problems, it likes to think you meant to select/drag'n'drop when you just move the cursor and try to click. If you need ultra light, this is the way to go.
Thinkpad's battery life is a bit disappointing, an hour or so less than Samsung's. The screen in general is better, the gamut is almost full sRGB. It's a very solid performer in all aspects. But it's much more expensive; you can get two (cheaper) Zenbooks with the price of one X1 Carbon.
Zenbook (UX31A and UX32VD) has superior display because of the fullHD, almost full sRGB gamut, best contrast ratio and almost as bright display as Samsung. UX31A:s battery lasts IIRC longest out of these, or at least it's similar to Samsung's. UX32VD is a tad cheaper, has the same excellent display, but with power-hungry Nvidia graphics makes its battery last as long as Thinkpad's, so not that long. (3ish hours playing video.) Also, hard drive makes it suck compared to other laptops with SSD.
Out of these, I'd choose UX31A. Good battery life, excellent screen, good performance, light and thin, and not insanely expensive. IMHO not worth the extra 1000 euros/dollars for X1 Carbon.
Oh, and if battery length is the most important thing for a light laptop, MacBook Air 13" is the way to go. Its display is though worst out of these mentioned in this post.
Since you reviewed it recently, I'll assume you used the second-gen Series 9. I've got the first-gen running Linux, and it runs quite well. I can definitely attest to the screen being quite bright, but it is more then good enough for side viewing (although the 1366x768 resolution isn't that great, but usable). At least under Linux the touchpad works perfectly though, much better then any touch pad I've used under windows. Probably the best thing about the first-gen though, is that being a pre-ultrabook, it has removable memory, so I've currently got it ramped up to 16GB.
The Zenbook Primes are not actually half the price of an X1 Carbon in the U.S., it's more like 75%. Also, the MacBook Air 13" has the best display for programming because it's physically taller than all the other 13" or 14" netbooks' 16:9 displays.
I considered all the laptops you mention and settled on the 13" Air. What's wrong with its display? I think it looks great, and seems to have a very wide viewing angle.
Compared to the others: bad gamut (color reproduction/range). Also, even if it's better than the typical low-resolution 1366x768, it still isn't 1600x1900 or full HD. Brightness- and contrast-wise it's great. But still, there's better displays in the cheaper price range already, like on Zenbook. Or if you want to compare against much cheaper iPad, iPad also has far greater resolution, much better (almost perfect sRGB) gamut, better contrast, and so on. MBAir's display is better than average though, so it's not bad. It's just there's better options available.
Of course, if you're not interested about the color reproduction, this might not affect to you. But if you want to edit photos or see movies like they're supposed to be, then it matters.
Samsung has the best battery-per-weight ratio, but it has the worst screen out of the three. Still better than average, except its gamut which is typical = bad. The screen is the brightest out of three and 2nd best in contrast after Zenbook. The mouse has some problems, it likes to think you meant to select/drag'n'drop when you just move the cursor and try to click. If you need ultra light, this is the way to go.
Thinkpad's battery life is a bit disappointing, an hour or so less than Samsung's. The screen in general is better, the gamut is almost full sRGB. It's a very solid performer in all aspects. But it's much more expensive; you can get two (cheaper) Zenbooks with the price of one X1 Carbon.
Zenbook (UX31A and UX32VD) has superior display because of the fullHD, almost full sRGB gamut, best contrast ratio and almost as bright display as Samsung. UX31A:s battery lasts IIRC longest out of these, or at least it's similar to Samsung's. UX32VD is a tad cheaper, has the same excellent display, but with power-hungry Nvidia graphics makes its battery last as long as Thinkpad's, so not that long. (3ish hours playing video.) Also, hard drive makes it suck compared to other laptops with SSD.
Out of these, I'd choose UX31A. Good battery life, excellent screen, good performance, light and thin, and not insanely expensive. IMHO not worth the extra 1000 euros/dollars for X1 Carbon.
Oh, and if battery length is the most important thing for a light laptop, MacBook Air 13" is the way to go. Its display is though worst out of these mentioned in this post.