I believe iPhones, Androids, etc. can use IMAP over SSL with GMail. There's a "Require SSL" or "Always" option on these devices under the mail account settings. Doesn't address your secure storage question though. For that, configure device authentication / PIN code to access. If it's a corporate-owned phone/device, you may be able to push/enforce security policies centrally.
Of course, there's a known bug in the iPhones that makes all that moot with certain versions of Ubuntu 10
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