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Its usually just whenever without any kind of trend or timeframe.

Okay cool.

Am I correct in thinking that once a particular BB version is on a phone, there's no way to downgrade to an earlier version?
 
You are correct.
You cant downgrade the BB unless on a certain limited bootloader version on the iphone 3G.

Now I might add that I have an iPhone4 with shsh's saved as early as 4.1 I'm not sure if that makes any difference or not.
 
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