I switched from AT&T to T-mobile when I got my N900, but not for about a week. I popped my at&t sim into the N900 and it worked with no problems. You can't do that with verizon, but you should be able to turn it on and use it with just wifi. If it makes you feel better about it, I did the same thing with my old AT&T phone the other day...I fired it up without a sim in it and used it with wifi.

If you use skype, you will even be able to make calls on your n900.

As for your dilemma, why not either:
1. just call the land-line number for verizon to get your voice mail messages
2. Go to a verizon store, put your sim in a phone there, and listen to your messages

Why wait?