The past two times I've started the bike up when it's been sitting overnight, the first 30-45 seconds of running there is an inordinate amount of blue smoke, then after that 45 second window when it gets warm, it stops, and I don't see any smoke. Is this relatively normal?

I should have another head heading my direction, so I could swap it on, but if I don't need to, I'd really rather not.

Sounds like worn valve guides, oil seeps down into the cylinders during idle periods and then burns away when you start the engine.

Best way to find out if it's a problem is with a compression tester. Let the bike warm up and check it. If the numbers are low, add a teaspoon of oil to each cylinder and check it again. If the compression doesnt go up, it's your valves.

Say it is the valves? Think its a big time issue? Can I just top up the oil frequently and ride off into the sunset, or do you think it will continue to get worse?

It won't get any better, but then again it's not likely to get much worse for a long long time. I wouldn't mess with it, whether it's rings or valve guides it's likely to have many thousands of miles left in it---like you said, keep a close eye on the oil level.

i have the same problem right now, but im just gonna top it off and retorque the head bolts because i just did a rebuild/overhaul, and im getting near my retorque interval for a rebuild, so im pretty sure thats the problem.  it could be a worn gasket on your bike though too, and once the bike is warm the metal is expanded enough to seal properly.  one thing you could do (if you dont have a compression tester) is warm it up good, shut it off, wait 10 or 15 mineutes, and start it again.  if cold is the issue, then it wont smoke when you start it. if valve seals is the issue, it may still not smoke because of expansion, but if it does smoke, you can take a good bet the seals are pretty bad.

I would suspect the stem seals before anything else. They are not expensive to buy but because replacing them involves removing the valves from the head they are often ignored during any overhaul.

http://motorcycleproducts.co.uk/catalogue/valve-stem-seals-suzuki-gs500-198893-p-134589.html

They're offering them in packs of 10.  Enough for 2 1/2 engines?

Buy em seperately here if ya want 

http://www.wemoto.com/bikes/Suzuki/GS_500_ET/96/picture/Valve_Stem_Seal_Exhaust/

I might just be due for a retorque. I've got under 1000 miles on this overhaul, and I did ignore the valve guides. I'm not going to worry about it for now.

man, thats a bummer, always do guides on an overhaul, wheather theyre bad or not, cause theyre cheap and easy to replace when the engine is already taken apart anyway.  oh well, hinds sight is 20/20. or sometimes 30/20. you can get some pretty good vision looking back.