But the reverse is true. If you bought music from iTunes you can't play it elsewhere.

Since iTunes are a convenient way to purchase music, some of us ended up with large quantities of music tied to Apple's devices.

Both the Wii and DSi can play the format iTunes currently uses. I think the PSP and PS3 can as well. I’m also pretty sure Archos and a couple other of the high end media players also work with AAC. AFAIK, only the low end stuff, like SanDisk’s fare, are still using silicon tied to MP3/WMA.

Isn't AAC an open standard?

Yes, although the silicon was slow coming. For a while, the iPod was the only game in town. The few other devices that used AAC at the time (like a Sony 8cm CD player I used to use) used the older MP3 style container instead of the modern MOV/M4A wrapper.

You know, I can't remember the last time I bought a DRM track from iTunes, and trust me, I buy a lot of tracks on iTunes.