Skype doesn't have any SIP gateways as far as I
know, though they're supposed to offer something called
"SkypeIn" this summer to do that.
they did a protocol of their own, and they will never publish
it, I guess...
Personally, I'd rather
just avoid them altogether, as it seems what they've
brought to the mix is primarily marketing.
Their protocol has the advantage that it works very well even
on low bandwith connections.
And what they did very well was to make their software easy to
use on every platform. Simply visit the homepage, download it
and a wizard will help you configuring it.
While the others still discussed SIP they did the market -
they simply did it clever enough.
Best regards
ce