The modem asserts a wake pin when you receive an SMS or phone call, causing the SoC to wake up. Services that rely on data (IM, e-mail etc.) don't wake the phone up (they will rely on periodic wakeups in the future, but there's no infrastructure for it yet).
An update that makes communication with the modem after resuming from system suspend reliable has been deployed today (previously it would often fully reenumerate and cause various troubles in the process, now it's instant).
