Is the way T-mobile stored Sidekick data standard to most cell phones or unique?

OK, my wife has a Sidekick, has had it for 2½ years and likes it quite a bit, or did until they lost all her data. So, basically as I understand it, ANYTHING on the phone, from your address book and contacts, to your calendar and notes, to the pictures you take is stored remotely? I always thought it was either resident on your phone or somehow tied to your sim card, for example, when I switched phones recently, I kept my phone book by moving my sim card to the new phone, and my pictures, even though the phone has completely discharged, if I were to charge it, they’d still be on my old phone (I thought that was the case). But apparently even pictures she took on her phone are gone.

And what I really don’t understand is, 2½ years ago when she switched to this phone, she put the sim card from her old phone in. Then she spent several hours editing that information so that it conformed better to how the Sidekick displayed information. She basically took her old contact list and redid it, plus all the info she’s added, and that’s all gone. YET, the OLD data from her sim card, the stuff that she originally brought over onto the phone is there.

So do most phones still store all your data on a sim card and this is just a Sidekick thing to store all data offsite, or is it common to have all your data be remote on your phone these days? I think I can probably speak for a million T-mobile customers who are really pissed off and who are NOT going to be appeased by one free month of service that we didn’t even KNOW that there was a danger that our data could disappear from a server somewhere.

Have I been understanding how data storage on phones works this whole time, or is this really freaking strange?