Honestly, in my opinion, probably not for awhile. The Arrive was just realeased. Sprint will try to get what they can out of this device before they introduce the next one. I'm not saying it will take forever, probably just six months or so, but I wouldn't hold off on getting the Arrive right now.
AT&T has had the same three WP7 devices for just shy of five months now, and just a week or two ago the HD7S was announced, without a date set yet (I think?).
I snagged the Arrive now, and will wait for the Nokia phones to replace it. The Arrive is actually a pretty good phone. And a quality stand-in till the Nokia partnership nets an awesome phone.
I sure hope we hear something soon. The only thing I have heard is that there is an HTC event April 12th, same day as MIX 11. They are expected to annouce some more Windows Phones. Who knows if any will be for Sprint though, keeping my fingers crossed. They need to get their act together fast.
But yea, I am waiting until Nokia too. Will just struggle along with my BB until next year I guess.
I just read an article on phone arena about the HTC Mazaa which looks like a rebranded HTC Trophy. I didn't post the link because I am not familiar with the website or the rules on links, but the article said that it could potentially be a World Phone.