Im currently waiting for a Haswell rMBP because its literally round the corner.
I wouldnt wait a full year for an update. Seriously just buy the machine and enjoy it. Every year something better will come along, thats just the nature of technology. You can wait a year, and you'll hear rumours of the developments for the following year, which will be much better etc. Its a continuous cycle. Where will you stop?
It's actually looking more and more it's going to be a 2 year wait. Intel's claiming having 'issues' with 14nm production so there will only be a Haswell 'refresh' next year with bumps up in frequency. IMHO, I think Intel is just delaying Broadwell due to lack of competition as pathetic AMD can't even hold a candle to them in performance and power usage. Get your Haswell MBA now and enjoy the 9/12 battery life for the next 2 years!