iphone
also, if your iphone was "exchange active sync'd" against Exchange 2007 or newer, you can remotely wipe it (brick it) rather easily. log into your webmail app (owa) go to Options, and look for the device heading.
i agree on the other recommendations:
1) sync anything valuable on there to a system not on the phone so you can "burn it" remotely without pause and your data stays unharmed.
2) use the startup pin # (and do check the box for "after 10 wrong attempts wipe phone") again, your data is elsewhere and you won't mess up so many timess
3) get the free find my iphone app (works on ipads too)
4) do encrypt anything "text like", check out "keepass", not a volume tool like truecrypt, but great for lots of numbers like s/n's
i have a balistics calculator on my iphone 3g and enjoy it, and it syncs my kindle books, handy for when in line at the (postoffice, store, coffee shop, etc).
anyways, most of those are "before the fact" - but the OWA wipe no one had mentioned so thought i'd add it as that provides one way to neutralize it without having had done any prep, as long as your EAS'ing (exchange active sync).
and for you old timers, why? access to data, period. s/n's is data pure and simple. with the tools done right (the encyprtion, and remote wipe) it's safer than in your safe in that a fire or thief can wipe out all of yours at once. sure, keep a 2nd copy at the safe deposit box - exactly what we're saying, but for us it's a click and a sync, not a drive down to the bank and meeting with a safety deposit box helper. anyways, not knocking your method, just saying when done right there are other methods out there.