If you just choose the "standard" DRM, you can end up with what happened to Microsoft customers and their MP3 players. "Apple is proprietary! Get PlaysForSure." It was a real good pitch, up until your Dell Whatever stopped being able to get new music because MS had abandoned PlaysForSure and Dell wasn't going to update their old players.
Amazon and Apple have large incentives to keep their DRMed content working on older devices, at least for a couple of years. Adobe has very little incentive to ensure that their DRM keeps working on the various Sony (and other) Readers.
Apple's tying of iBooks at least partially to iOS updates actually makes them the worst offender in this area. I'm willing to bet 1st gen iPads can't get some new iBooks, but I bet all 1st gen Kindles can still buy all books from Amazon.
You lose this bet. There are quite a few Kindle books like this: http://cl.ly/image/2t2m2V3Y0513 that won't play on basic Kindle devices. I discovered this when I tried to read one of my books on my gen 1 Kindle.
There are also some unexpected restrictions on some titles, such as books that are viewable only on 1 or 2 of your devices, which is quite annoying to discover when you're not near one of the previous devices to deactivate it.
I can't comment on the functionality of iBooks on Gen1 iPads because I've never purchased an Apple iBook, but your guesses about Amazon are, sadly, wrong.
You can check this when purchasing on Amazon: the small print you're looking for in the Product Details section is "Simultaneous Device Usage: Unlimited". If you don't see that, it means your Kindle book is restricted (I believe to 5 devices simultaneously, though perhaps it varies from book to book.)
It's not an optimal solution, but it's mostly OK.
2) Use Calibre to unlock the .mobi files, using the serial number (conveniently located under the "Device Settings" or "About this Device" menu - I forget which it's called, but it's a ~16 digit number and very easy to find.
3) Upload to S3
This works on Linux - I assume it works on OS X and Windows too.
> And of course Kindle and Apple customers won’t even notice, thanks to those companies’ wise decision to use their own DRM.
About the only thing "wise" about it is that if Apple were to change its own DRM scheme for the books it sells, I'm assuming they'd update iOS to include the new functionality. Admittedly, this would still lock out older devices potentially that couldn't upgrade to the latest version of iOS.