The Google Talk client is horrible. It wastes tons of CPU when it's loading up the contact list, and still uses a bunch just while idling. If you have it on all the time, it will eat your battery away such that a Blackberry 9700 with no other apps running won't last a full day. I tested this on my own device a while ago, it might have gotten better, but I doubt it.

Blackberry Messenger (BBM) also has a typing indicator, delivered and read indicators, and it's faster than anything else out there.

Also, the fact that it's only on mobile devices and not desktop computers means that you know your message will get delivered to someone's pocket instead of the computer they may have left on. An odd side effect of its lack of portability.

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.

BBM = Mobile - Mobile Messaging

GTalk/MSN etc = Mobile - Desktop Messaging

The use cases are completely different.

What if you run gtalk on your blackberry, and I run gtalk on my android phone. Is that still Mobile - Desktop?