According to a reddit user, Google has begun sending out invites to Project Fi, and while the entire process appears to be straight-forward, a possible roadblock comes to us in the form of a Google Voice decision screen:
If you happen to prefer your Google Voice phone number over your current mobile one, you'll be able to adopt it at this page. Afterwards, Google will transfer some data from your Voice account over to Fi, including voicemail greeting, voicemails, texts, and history. The caveat comes next: by moving to Fi, you'll lose the ability to receive calls through Google Talk or Google Voice, and you'll also lose the ability to use Voice for texting and voicemail.
For some, none of this may matter, as it's not as though anyone will lose the ability to send texts and voicemails -- it will just be through a different service. What this probably means is that you'll be forced to use Google Hangouts for any sort of data communication.
Since Project Fi is just rolling out, we're bound to learn a lot more about this drawback soon, and once the service is available to more Nexus 6 users, we'll be able to see more details flesh out and see if it really is as good as it appears on paper.