mrf49xa

These two chips are seen as very similar and their datasheets have very similar figures.

MRF49XA http://www.microchip.com/wwwproducts...cName=en541404
Si4421 http://www.silabs.com/products/wirel.../Si442021.aspx

Does anybody know more details about these similarity?

Add the TRC101 by RFM to the list, it's exactly the same. I'm curious about this too.

I don’t think these manufacturers follow a pin-to-pin compatibility, or a standard in this sense.
I know that Microchip, which originally was just a digital microcontroller developer, made agreements with some RFIC manufacturers to use their RF transceivers in the same package with their microcontroller. But now I think they have their own RFIC design division.