I don't think the sell-through on the iphone has been lower than expected. Unless Sprint is out right lying on its earnings calls, which probably isn't a good thing for them to do, then they've matched or exceeded their sales estimates for the iphone. The iphone has been their number one selling phone since they released it on sprint. I'd be curious to see if that's still true with the EVO LTE now out.
And of course once the next iphone is released and it has LTE onboard, then phones will fly off the shelf once again for a quarter or two. I"m sure Apple and Sprint will show record sales of iphones in the quarter following the release.