It's just that... a rumor. It is likely that RW will offer new phones later this year. But, no announcement has been made, and if any new offerings are near, RW is holding it's cards close to the vest. You can wait till Summer, but the new phones may not be offered until the Christmas shopping season, or next year even. Only RW knows, and they're not telling. The Defy XT is not a bad phone. The newer phones will likely cost a good bit more, too.
I doubt there will be a trade in credit on any republic phone, but it is an unknown at this point. The following faqs don't specifically answer your question, but may be useful to you:
If I purchase the Motorola DEFY XT now for $99 and pay $29/month, it looks like Republic Wireless will recoup the cost difference within 15 months. After 15 months, can I switch to the $19/month plan?