Some background on the specs: this phone is a pretty high end unit, including a 4GB hard drive (store tons of music, photos, etc.), standard headset jack (use your killer headphones to listen to the music), 2MP camera (almost decent enough quality to leave your low-end digital camera at home) AND, the big one, built in Wi-Fi of the 802.11G flavour.
It's this last that is interesting. Nokia has already said that is working on Voice over WLAN: see Time Europe -- "2006 will be a big year for [mobile] wi-fi" and vnunet.com -- "replacing mobile and desktop phones with series 60 Nokia smartphones could make IT management easier". So, my guess is that they are specifically holding back the release of the N91 so it can be one of the flagship phones to include this functionality.
Now the other big question is: will Nokia work with Skype to include a Series 60 compatible Skype client on the phone, or will they do a standards based SIP client? If I were advising Nokia, I'd tell them to stick Skype on there -- I have to believe with the eBay purchase that users are more important than ever, more so than fighting the carriers, so this might be a doable deal today.
Of course, there is a SIP API for J2ME and an existing project to build an open SIP stack for mobiles, and Nokia should probably make sure this is available as well if they are serious about pushing both Series 60 and open VoWLAN implementations. Scott specifically pointed out SIP support on his post about wanting the N91, so it looks like that is a done deal, but you really need some sort of service provider to make this useful to the average end user (perhaps average end user is a meaningless context in this case, since Scott already runs a full VoIP infrastructure at home).
Supposedly this phone was designed by the same team that built the N-Gage, who also happen to be right here in Vancouver. Roland is still trying to get us some early access to phones. Anyone have strings they can pull at the Nokia labs here in town?