You do realize that opinions will vary widely here, right? Further the MB is not in anybodies hands yet and thus we don't really know how well it will perform. The big concern with the MB and its fanless design is the fear of thermal throttling. We simply don't know how bad it will be nor how quickly it will happen. So a definitive answer isn't even possible, further you would need to find an XCode developer actually using the MB for development.
Was looking at a base MBA, but the appeal of the screen and extra RAM/SSD in the MB is very tempting. Have no issue with the adapter, somewhat prefer that to hooking up numerous cables. Aware the MB will cope with the majority of my Chromebook-esque use nicely, but what about iOS development when hooked up to external monitor? Perhaps some Sublime Text but then I can use my PC for normal web dev. Somewhat looking forward to the novelty of SSH without Putty.
Text editing with a decent editor shouldn't be a problem. However if you intend to run XCode you may not get the performance you want, can't say for sure though so I'd hold off decision making. The problem is XCode can be very demanding when everything is running, lots of processes and memory usage. The other thing that may be a problem for you is the screen size and there XCode can take up lots of screen real estate.
As a side note XCode use to be far worst resource wise that it seems to be in the last release. I was running not on an old 2008 MBP and version 5 was a true dog on the machine. Hey must have done something right with with version 6 as it ran much better up until the MBP dropping dead a few weeks ago. On that same machine I use to also run Eclipse and PyDev, Eclipse actually felt better than XCode. So the question is can the new MB give you the same performance as that old machine running XCode. Well it should as long as XCode doesn't put it into thermal throttling conditions. With Playground and other XCode features all running at the same time I just don't have the info to answer here. With all the RAM and the turbo boost capability it should perform better than that old machine.
The problem is XCode is so sluggish on that old machine that that isn't saying much. I'm still of the Mind that if you want to do anything more than trivial development that you will want a more robust machine. Mainly you want support for threads and high clock speeds that are reliable.