What has changed with plasmas since my PX500U purchase?
Picture quality has improved, electricity consumption has decreased, 1080p has become standard and sizes have gotten bigger.
Have LED and LCD technologies caught up with plasma as far as picture performance and viewing angles?
They have improved but mostly, no. They can use interpolation to achieve higher frame rates to smooth motion and prevent jerking but that creates a very unnatural "soap opera" effect. LED backlighting can greatly improve LCD black levels in theory but in reality quality models are few and costly and cheap edge lit LED LCDs can be behind the fluorescent models they replaced in some respects. LED LCDs can strobe the backlight to improve motion but some people hate the flicker. There are some IPS LCD models with better viewing angles but usually they come with relatively poor back levels.
An LCD probably isn't what you want if you want a plasma substitute.
Is Apple close to launching their TV? Sorry if that is taboo around here.
We don't know but probably not. If they do it'll be a rebadged Japanese LCD with Apple case and interface.
Is there a mfr that stands above the rest with plasmas these days? When I last purchased Panny and Pioneer were the clear leaders.
Panasonic has opened a clear lead this year, the only exception being in the area of image retention where Samsung is ahead of LG and Panasonic. Image retention does seem to be a sample variation thing with some individual TVs being much better than others. Some people prefer the way Samsung handles standard definition upscaling but this is disputed.
Unless you're in a basement the thing to look for is a plasma with anti-glare coating. The U / UT series Panasonic plasmas don't, the ST, GT and VT do. What model's right for you really depends on what you want to do with your TV.
If you're in no particular hurry you could wait until the 2013 models come out.