Time to upgrade from my 2009 MBP? It's so hard to "throw away" a perfectly functional laptop that does everything I need - just not as well (I realize) as a faster, lighter, machine would.
If you want the new shiny, just sell your old MPB on. Stays out of landfill, makes someone happy &c. Else, just 'Use It Up, Wear It Out, Make It Do, or Do Without'.
Is there anything your laptop can't do that is important to you?
I would wait a bit for reports of how well the retina displays are doing. I have a MBP 15" that I need to take back in to get the display replaced since it ghosts the old images for minutes. Also, I am curious if the chipset drives the display decently.
There will probably be multiple suppliers of these displays, like there were for the Macbook Pros. If the new panel is IPS and from LG, it will probably have the same ghosting problem. (If you have a LG IPS display, you can see this issue clearly by watching a Youtube video in a window for 15-30 minutes and then immediately loading Metafilter in the same window.)
Actually, I would buy right away. Apple generally sources higher-binned (higher quality) components early on. i.e., only Samsung panels when the Retina MBP first launched in 2012.
Broadwell will be perfectly capable of driving the display.
I know what you mean my Vostro from a few years ago is still a decent spec (i5-2430/16Gb DDR) however I found out today you can now get the X1 Carbon with the 2560x1440...my Vostro is looking more and more like it needs replacing.
If you're a developer - take a look at the keyboard first. They replaced the F keys with capacitive buttons (ie. touch sensitive buttons as opposed to physical buttons) and the capslock key was split into a home/end key instead. Sure it's still a nice laptop, but that sort of thing would drive me crazy.
I would check out the X1 Carbon in person before making that decision. The display is terrible. It's the only device over ever used that's given me significant headaches after even moderate use.
Is there anything your laptop can't do that is important to you?