I posted about the Rab on-demand pullover, as the 2nd post in this thread, but again a good write up of the on-demand pull over can be found here
http://www.outdoorsmagic.com/news/article/mps/uan/6205

Its 9.87 oz, in what i presume is size medium. So you get a lighter version of the ID thru-hiker, at the cost of half a zipper. (it looks like a very deep zipper though) However, you get a better hood than the ID, which for some, will offset the cost of only having a half zipper. That and the lighter weight will make it a win-all for those same individuals who don't mind the pullover/anorak style. I should also note that the Rab lacks the velcro adjustable cuffs found on the ID thru-hiker, and has instead taken the elastic cuff route, ie Marmot Ion, or 09' Marmot Essence
As for comparison to the ID Rain Jacket (the other option that ID offers), well the Rab is heavier but offers an actual real hood. The hood on the ID rain jacket IMO is a joke (albeit, i have not tested one, just based on my observations regarding the design of it).
Rab is a great company, based outta the UK. Like Brandon mentioned, their event jackets include the drillium, momentum. Their mountaineering jackets and pants, gloves and gaiters also use event (ie. latok, latok alpine, Bergen). They are also one of the first to adopt the newer waterproof pertex fabric (same stuff as Golite's Tumalo Line). That being said, their production is still overseas, in China.
I personally own a Rab drillium, and have written about it in the past. Its a company that I would buy from again, w/o hesitation
Also, i clicked on Roleigh's link to Rab's official website, and description of the on-demand pullover and saw this, under the description
"Popper at top of collar allows for venting options"
What is a "popper" and how does it work?
Also, OMM is coming out with some even lighter eVent jackets. Check out this thread
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=28254&disable_pagination=1
More at http://www.outdoorsmagic.com/news/article/mps/uan/6636

At 8.46 oz, and what I consider to be a better cuff system, this is a winner over the On-demand pullover. But considering that I'm never going to get my hands on one here in the US, it is nothing more than a dream :/