It means that aspect ratio is an only difference between NTSC and PAL ?

I think "Maintain aspect ratio" is the default, so it probably doesn't need changing. But if you do, be careful with Paste Attributes since it copies all attributes (FX and keyframes and so on).

True indeed, unless he had pan/crop previously assigned, he'd still need to correct. And I'd failed to mention this will copy all attributes. thanks for pointing that out.
Voytek, frame rate is different, but Vegas already knows all of that...

Same project 1hr 50min
rendering time;
to PAL DVD - 18 Hrs
to NTSC DVD - 3Hrs
Is this normal

Sounds like frame rate resampling due to 29 > 25 fps - this will make a huge impact on rendering times.

This can be turned off on a clip-by-clip basis (or select them all at once and turn off): right-click on vid-event>switches>disable resample.

I regularly go the other way, just by rendering to NTSC mpg. Haven't had to change AR, maybe I should carefully check the final DVD to see if it's OK, actually that may explain something, I think the circles were coming out oval, oops. Thanks SPOT!

As to render time, yes, they'll be HUGE. Still time costs little, feeding your source through a standards converter that'll do as good a job as Vegas will COST.

Am I the only person then to think the quality of Vegas' conversion from PAL to NTSC is absolutely and utterly appaling?

You must be.....Vegas does a better job than my Laird converter does, and it's a whole lot more money.
When I was on the VASST tour in Australia, the Canopus folks were shocked to see what Vegas could do with NTSC for PAL. All my training media was NTSC, but I needed to display it on PAL monitors. If it's good enough for very high end conversion companies to use in Hollywood, I'd guess it's good enough for most.
What specifically do you find troublesome?

I can only second what SPOT is saying, I've never had to do a NTSC to PAL conversion, certainly that's never going to produce pristine PAL becuase you're starting with a lower res source but going PAL to NTSC looks about as good as NTSC gets from what I can see.

I'd imagine the very best broadcast gear that includes motion estimation should get better results but that kit is very expensive and unless your material has a lot of fast motion I doubt you'll see musch improvement. Also of course it's probably not starting of with DV material as well.

I have seen some suggestions around this forum for some very complicated ways to do the conversion, I've never tried them but as they involve so much fiddling with things maybe that's where you're getting a quality loss. I just do a 'Render As' and all is sweet.