I suspect the lack of notifications is due to Skype's firewall-busting P2P-based system. Tango [video call app] uses mere XMPP.
It's tough because, if I understand correctly, Skype facilitates peer communication but then lets the peers talk with themselves.
So where do you put a notification server? Skype will need to run as a 24/7 service on the phone to give you notifications, which isn't available on WP7.
Skype delivers a few bootstrap servers, handles login, the rest is on P2P.
I think Skype's inability to keep a persistent network conn. also prevents notifications. It has to punch a hole to talk to the outside world. If it can't keep that open, other nodes on the Skype P2P network can't call in.
Anyway, point is: It's easy to shit on Skype. (I know, I do it all the time.) But Skype is way more complex and different than you think.
Texts are easier; they're sent to Skype servers for storage (30 days max, they claim). Means easy to notify users.