I have had this problem a total of 3 times so far on a stock sprint rom- the only solution for me was to hard reset and reload a backup made by Sprite. Some observations:
1) Its has happened to me when I was either roaming or had bad coverage
2) The phone occasionally seems to work but touchflo does not load- toggling it on and off does nothing
3) When I am stuck in this "mode" I get the "hands free activation" message- when this is displayed the phone's battery gets discharged fairly rapidly
4) If I try to soft reset it several times I will get the message on the grey screen
(see here for picture)- the phone cannot boot etc
Like I said it has happened three times so far- unfortunately I am too lazy to rebuild the whole phone with all my programs so eventually I hard reset the phone and load the Sprite backup from the night before- this means I never really start "completely over"
I wish I knew what was causing this- my hope is that a fresh build with the upcoming 6.5 stock rom will solve the problem