Basically, if you use a 8-12 digit numeric passcode (random), and an iPhone 4S or iPad 2/3, your data will be safe from cold boot attacks for between 4 months and 3000 years. You still may suffer from legal or extralegal compulsion to turn over a passcode to unlock the device, evil maid type attacks, (or unpublished backdoors or vulnerabilities, or hardware attacks on the security processor, but these are beyond local law enforcement and probably sufficiently sensitive that only intelligence agencies would use them, and only when an alternate source could be plausible).
That's about the most secure computing device in default configuration ever shipped. Pretty amazing. The only reason forensics can generally work on iPhones today is that 4 digit numeric passcodes are the default, and "delete on 10 tries" doesn't get enforced correctly on current generation hardware. If they fix that on the iPhone 5, then 4 digit passcodes become reasonable.