Since intels can boot from USB, it's not surprising that a malfunctioning USB device could stall the boot process.
Are there any devices plugged into the usb hub of the display? try unplugging them.
Ok... THAT just blew my mind. The only thing that was still plugged into the display's USB was the charging cable for my Jawbone 2, which I hadn't yet moved to the tower since this started. I also had only just started using this Jawbone again the other day after losing my preferred headset. I plugged the display's USB cable back into the Mac, restarted the tower, and with the charging cable and the Jawbone in place, the Mac wouldn't boot. Instead of unplugging the cable, I pulled the Jawbone off the charging end, and the Mac booted right up immediately. That pretty much left me dumbstruck. I coulda guessed at a USB stick perhaps as being a potential problem... but certainly not a headset.
Oddly (to me anyway), the Mac will boot right up just fine if the Jawbone is charging from the cable plugged directly into the tower rather than the display. Go figure. Well anyhow... this is all a relief. My wife wasn't about to go along with getting a new iMac now and instead was looking at some #$!& cheap HP monitor to hold us over for another year if this display developed further issues. *whew* Thanks for the tip!
