I wouldn't get too excited. This only seems to be the addition of S/MIME client capability for Google corporate email customers. So the Google corporate customer will still have to procure, renew and pay for a S/MIME certificate for each and every employee. That isn't something that companies have shown much interest in up to this point.

This does not affect non-corporate Gmail users...

This is also cool because unlike a lot of other end to end systems you don't have to trust the client they provided you with.