For the uSD card the magnet thing will just work. It's designed to work that way, that's why there's this magnet and hall switch. For SIM however there's no notification to modem about magnet removed (on hindsight I wonder why they didn't do it that way. I have no proper datasheets about bb5 modem chip but I'm sure it has the standard card-detect input pin, Nokia could have hooked up this to the hall switch just like they did for uSD, and place the SIM tray somewhere where you could swap without removing battery)

@estel: sure sync && umount and wait till finished is a 100% safe measure to avoid damage to uSD card. For SIM there's no umount or sync I know of, but that doesn't mean it doesn't exist. Maybe some magic AT cmd will do exactly what we need, AT+CFUN=0 or whatever. Anyway dual-SIM-adapters afaik power both SIMs in parallel and without interruptions, they simply switch the CLK line between card1 and card2, and obviously they can check for activity on data line before they do.
But me is not the dual-SIM-adapter expert here ;-) I'm sure we will hear more experienced explanation on what exactly they do, pretty soon.

sidenote: it's not surprising that you don't need SIM during phonecall, the SIM is only needed for authentication. Once you're logged in to the network, SIM isn't needed till next re-auth which , I guess, will only happen on expiry of T3212 timer, i.e. usually several hours (<-this is straight from my a..., I'm just guessing, except for usual duration of T3212. Maybe re-auth gets triggered by other timers, other events, or not at all. T3212 is for the location update).

sidenote2: it's actually quite concerning that the modem immediately detects the swapped SIM. This means the card must have been active, and that implies you could have interrupted some write-cycle. Sounds scary. And I don't get it. For power saving reasons the modem is supposed to power down the SIM after short while of idle, and there's no reasonable way to detect removal / insertion of a SIM from/to a powered-down tray (that has no dedicated switch for that). Hmm, on second thought there *might* be one or two ways (weak pullup R [100k] on VDD)

/j