It looks like Boost Mobile will be getting a new BlackBerry soon. It's the Style 9670 flip phone which has been available to Sprint customers since late last year.
The Style hasn't been announced by Boost yet, but the news was leaked by BestBuy including an ad (image) for the new phone in their July Mobile Phone Buyers Guide. If you want to see the ad for yourself, go to BestBuy's Weekly Ad at bestbuy.shoplocal.com/bestbuy/default.aspx?action=entryflash&adref=header and scroll through the "Available Ads" picker in the upper right of the screen until you find the Mobile Phone Buyers Guide July. The Style is on page 26 of the guide here in the San Franciso Bay area but may be on a different page in other markets.
The Style is a nice upgrade from Boost current BlackBerry, the Curve 8530. The Style has a beter camera (5 MP vs 2 MP) and a slightly larger (2.7" vs 2.5") and much higher resolution (360x400px vs. 320x240px) screen. It's also the first prepaid BlackBerry with OS 6.0 whose main feature is much improved WebKit based web browser with very good HTML5 support.
If you are interested in the Style their are lots of reviews of the Sprint version on the Web including ones from PC Magazine, CrackBerry and PhoneScoop.
Unfortunately the $279.99 price shown in the ad seems high for what you get. BestBuy as a tendency charge above list price on non-sale items. Perhaps the actual price of the Style will be lower when it's released.
Update: The July Boost dealer newsletter (PDF) has a page promoting the BlackBerry Style and saying that it's "coming soon" with a MSRP of $199.99.