Awhile ago some crafty people were lurking around their device and found out that going to "Dial Pad" and typing in ##EVDO# (aka ##3836#) brought them to one of those secret programming menus.
The first thing you see is a big ol' checkbox that says "Rev A" unchecked. Of course people flew into a tither but in reality, a lot of did not see any increase in our download speeds (via Mobile BBR)--in fact, a few people have experienced some odd behavior with it checked (mostly because they didn't choose "Automatic").
Step in kaptainchump who decided to check upload speeds and sure enough...that's the difference.
Without the "Rev A" box checked, upload speeds cap at around 120kbps. When enabled, you easily and consistently hit up in the mid 300kbps range (that'd be doubling your upload speeds in case you didn't notice).
Now, why is this not checked to begin with? Forum member Ebag333 speculates it might actually have to do with Sprint's programming, while others may think it's currently unstable.
Who knows at this point, but if you're feeling a little risky, take this trick for a spin.