Just so everyone's clear up front, Microsoft has just come out with a list of twelve things they don't want to see on the Windows Mobile Marketplace. It includes VoIP.
The list is below, but the major ones are these: no VoIP that goes over the provider's network (Wi-Fi is OK), no apps that make you download more than 10MB over the air (not Wi-Fi), apps that use/promote other marketplaces other than the official Windows Mobile ones, apps that make you pay money outside of the marketplace (which bypasses Microsoft's cut of the profits), and apps that publish your location without permission.
They're all pretty reasonable expectations, but not being able to have VoIP over the carrier's network is a step down from what it is now, where you can download VoIP apps and talk as much as you want. Looks like Skype isn't going to be distributed through Microsoft's Marketplace then, or at least just locked down if it is? [PC World via Fierce VoIP via Tech Digest]