I am a relative new comer to the Mac sphere of things. I personally bought my first MBP a bit over 2 years ago, and my current employer bought be a new MBP 3 months ago. These are the only Mac products I have ever owned. I didn't think I would like them as much as I do and I honestly broke down and bought my first one due to social pressure, to be brutally honest.
I have done a lot of thinking about the whole "when to buy Mac products thing". At first I almost didn't want to buy my first MBP because "what if they announce something new in X weeks/months from now and I miss out!" "What is new announcement in X weeks/months lowers the value of my current MBP!" "Ack I am going to be BEHIND!" These are all real thoughts that I had. Then I realized the flaw in my own logic.
First of all, the value of my MBP is not in its resale value. It's in my ability to complete work for clients which brings me $xxx dollars per hour (or really $xxxxx dollars per week, but that's an entirely different discussion). I have two MBP's in my possession at this very moment that are top of the line and exactly two years apart. If I placed them side by side and told you to pick which one was the newer one, you couldn't. If I let you code on them for one day each and then told you to pick the newer one, you couldn't (unless you looked in "About This Mac" but that would be cheating now, wouldn't it?). Trust me, the hype over the latest and greatest MBP feature is exactly that, marketing hype.
Second, MBP's are honestly just really solid machines for working. The battery life is long, the case is light, the features are more than sufficient for development. My 2+ year old MBP is running like a champ (and if it wasn't I would just take it to Apple, I have Apple Care, and they would fix it). Decide what you are buying the laptop for. If it's for telling your friends that you have the latest and greatest MBP that looks exactly like every other MBP, then wait until the newest one comes out and preorder it, then get ready for it to be old news three weeks later. If you want to build great iOS apps for your own business or for others, then get a good solid machine and start making money with it and stop worrying about all of the Apple hype.