shaftsburn, yes as I said SPD INT and ALT INT are just options and many aircrafts are not equipped with. The DES NOW you are talking about, if pressed before TOD under VNAV PTH pitch mode, will start the descent at a rate of 1000ft/mn and at FMC SPD speed until aircraft catches the descent path, then it will follow the descent path at IDLE power in normal VNAV PTH. you will always be under VNAV PTH during that time.
I would say that the SPD INT is like a shortcut to change your speed without manipulating the fmc but still under VNAV.
exteicate, as far as I know, and with the 737 NG, there are only 2 situations where you will get VNAV SPD during descent:
1- By pressing SPD INT on the MCP (if equipped) while under VNAV PTH (speed window will be opened)
2- If for any reason you cannot start descent at TOD: FMA will change from VNAV PTH to VNAV ALT first, then after setting a lower MCP altitude, it will start descent in VNAV SPD (normal because aircraft cannot maintain path anymore) and the aircraft will start descent at idle power at the descent speed set in the FMC