I really like SIP. SIP is the technology that lets you make calls over the internet often free of charge. There's one downside to it: not everyone has a SIP connection which is why many people use free providers like Skype. Before I got my N900 I rarely used Skype. I don't like being tied down to my computer to talk. I'm a bit of a peripatetic when talking and want to wander where I will. Hence my reluctance to use Skype.
I've tweeted a couple of times recently about using Skype for making calls on from my N900. Initially I used Skype because others asked me to. Usually it was for a conference call. Recently while sitting in a restaurant I received a Skype call. What caught my attention was that it was over my cell carriers always-on internet connection — not the restaurant's WiFi (the restaurant didn't offer WiFi). Very intriguing!
Later a took a Skype conference call on my N900. Taking Skype (or other Voice over IP) calls on my regular handset is great because I can use the handsfree set I have, use other in-call features and of course walk or drive away from my current location. And the N900 makes managing Skype contacts very easy — they're integrated right into the address book. It's just another data field along with cellular number, fax, email and IM details. It's fully integrated. When I touch a contact to make a call, a list of available calling options pops-up including Skype. If had Skype-Out minutes I could use those, presumably for cross-border calls to phone numbers. It would make little sense to use Skype for a US number unless I was running low on mobile minutes.
I'm finding that having a built-in Skype client is a helpful addition and make my N900 more valuable to me than it already is.