Typically Apple releases a new version of iOS when they release a new iPhone once a year (seems it will be each fall now, but that hasn't always been the case). You'll hear all about it at WWDC, which is typically in June, and they'll announce what products will be supported. The pattern seems to be fairly predictable: any product still being sold when the announcement is made will be supported. This will get you three or four generations of iPhones, perhaps two or three iPads, though the iPad line is still fairly young and more of them may be supported going further. The iPod Touch is probably the least supported line because it's basically using old iPhone hardware, but you'll get the last couple generations.
Now with the over-the-air delta updates, upgrading iOS could not be easier. Just go into your settings, hit Software Update. In a few minutes you'll have the latest and greatest, so long as you're not using a device more than three years old.
A few iOS features won't be supported on every device, but that's to be expected. No cell network stuff on devices without cell networks, or using of sensors on devices without those sensors...
I think you'll really enjoy the ecosystem, if you have any other Apple devices. If you don't have any other Apple devices, it won't be long now I'm sure, wahahaha!
But seriously, only Microsoft is really in the same position as Apple to have their software on all of your devices. Critics call it lock-in, but there are significant advantages. Having been spoiled by Mac OS X and iOS working together so well (not to mention stuff like Airplay, Apple TV, etc.), if Apple suddenly went out of business, I wouldn't go to Android, I'd go to an all-Windows/XBox solution. I wouldn't like it as much, but it'd be better than having such a disjointed experience straddling multiple platforms.
The haters are certainly a remarkable phenomenon. I like Apple products, but I don't seek out dedicated sites for other platforms and try to tear them down or just talk endlessly about Apple products being better. I'd rather spend my free time on more positive pursuits, and let other people use the sorts of products they enjoy in peace. All I can figure is there's some insecurity there, but where it's comes from I have no idea.
Anyway, welcome to iOS! I'm not getting the latest iPhone, but it certainly looks quite nice and I hope you enjoy it.