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It (insert whatever PPC IM app here) won't work like the sidekick because the sidekick IM is integrated into the phone with all data going through Danger's servers before it ever goes out to the internet. So Danger maintains your AIM connection even when having a call.

Well your carrier isn't going to do this obviously if they didn't pay to have the IM client built into the phone.

The only exception is text messaging based solutions because it doesn't require a constant data connection to stay connected. But the Mogul doesn't have OZ IM either.