I think now Skype has some real competitor. This is also going to add lots of value to Google Apps product.
Only if their network is more stable and call quality is equally good or better, and they hopefully offer an API... and expand it outside the US... and to Apps domains.... Skype has such momentum, and with partners like VoSky allowing corporations to tie Skype to their PBXes, it's going to take more than an integrated client to shake the tree.
I'm rather confused on why Google Voice isn't tied into Google Apps domains yet. Take Google Voice, add some management tools and say "Hey guys, for the $50/user/year you'd pay for email, docs, etc we can be your PBX too!".