As you level stealth, sometimes it seems like you're essentially invisible. But I've noticed that now that I'm maxxed, people seem to detect me more often. Is this just because the people are leveling, too? And is this just my imagination, or have other people noticed this?

Im outa town and I got my Skyrim pre order very late so I havent been able to see this yet. But I can kinda relate to this in FO:NV for a while it seemed like I was able to walk around and even have people walk right infront of me and not see me. Then when My stealth got to 100 and no matter what type of armour I wore its like my enimies could always detect me. Much of it also depends on what type of foe you are facing, some you can literally bumb into them and they wont notice you while others its almost like they can smell you..

I've noticed that the environment is disturbingly fragile as to being seen or not. I was hunting Thalmor and even though it was night they could easily see me. Just off of moonlight at 20+ yards.
Then it started snowing every so gently and I could walk right up to them our roll around the place and they'd be clueless. I wish their was a light scale similar to Splinter Cell so you could tell the visibility for NPCs.

animals can smell you, and sneak often doesnt work on them.

But for humans sneak is still super effective at any level.

Humans/Draugr/etc are pathetic at noticing a stealth player...the AI detection is actually several steps back from Fallout 3 and New Vegas.

You could be comparing stealth in dungeons vs stealth outdoors.

In dungeons you are very "invisible", but outdoors not so much.

I find that the falmer are really good at detecting. Though thats probably because of their increased hearing sense.

Most of the named civilian NPCs do not level with main character.

Guards and followers are leveled. Most of the creatures and enemies in the wild are leveled.