Apple have one phone, or a few phone models anyway, but pretty much the same hardware. So one OS works with minor variations on all of their phones.
There are, however, nearly 100 Android devices now. Each one has different hardware. There's several brands, several CPUs, and vastly different specifications and drivers.
Once Google release a version of Android, the hardware manufacturers then have to make it work with their phones, and often add or upgrade their own customisations like HTC's Sense UI, Motorola's Motoblur, or Samsung's TouchWiz.
Not all phones are capable of running the latest version of Android due to hardware limitations (usually insufficient storage and memory). Some Android phones are still stuck on 1.5, or 1.6. Only the Nexus One officially has Android 2.2 at the moment.
Once the manufacturer finishes getting Android ready for their hardware, the mobile network (if you bought it from them) will also need to customise it to add their own apps, graphics, sounds, etc.
This whole customisation process from Google releasing a version to it arriving on your phone (assuming you ever get it, depending on hardware) can take 2 to 6 months, and in one case nearly a year.
Also, where you live comes into effect, as different regional variations are also released.
So, when you get Android 2.2. will depend on your handset model, the network you purchased it from (if any), and where you live.
Most updates are pushed out over the air (your phone just tells you one day you have an update and offers to install it). But many places also let you download the updates from their websites.
Unless you hack your phone to bypass its built in protective security, the phone will only let you install official ROMs that are specifically for that phone/hardware/network combination. So you can't just grab a Nexus One Android 2.2. OS and expect to install it on an HTC Desire on T-Mobile, for example.
