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None of these are especially created for mobile. Or aimed squarely at BBM with the same features.

I had been thinking about creating BBM for other platforms simply because I'd like to have it on my iPhone.

How would you create a BBM client for other platforms? You wouldn't be able to authenticate with BB servers since you have no PIN.

WhatsApp is a near approximation of BBM on the iPhone (and other devices, I believe).

Their reliability isn't great, however.

WhatsApp has better reliability, more handset clients, more features, and more users.