If you're downloading large amounts of data directly onto an iPhone 4, you'll obviously use Wi-Fi whenever possible. But be cautious when doing so overnight: you might find your iPhone reverts to using your more expensive 3G connection instead.
Lifehacker reader Holger wrote in to describe how he'd been caught out by this unexpected behaviour by the iPhone 4:
It seems that the iPhone 4 will not persist a Wi-Fi connection when it sleeps. It drops back to 3G. I was burnt by this last night when, after getting my iPhone 4 and upgrading from my 3GS, I decided to catch up with some podcasts by letting them download while I slept (I use an RSS app for podcasts as I prefer to download outside of iTunes). I awoke this morning to not only find that my 3G download limit had all gone but the remainder of my prepaid credit ($60) on Telstra had also gone despite being connected to the Wi-Fi when I went to sleep and having to turn the Wi-Fi off when checking my balance when I woke up.
You may want to warn others, I only lost $60 because that was all I had on my account. For those on a plan where Telstra (or other providers for that matter) just rack up the charges, this could get very, very expensive very quickly. The workaround for the time being is to turn off 3G when not in use.
Turning off 3G will certainly avoid the issue, but obviously isn't ideal if someone tries to call you in an emergency. Experienced a similar problem? Know a more effective way to avoid those hassles? We're all ears in the comments. Thanks Holger!