Welcome to FT!
Could you list the specific leg(s) and date(s)?
8/15/07:
SQ491 DXB-SIN Availability: D1
8/16/07:
SQ806 SIN-PVG Availability: D1
8/16/07:
SQ375 DXB-SIN Availability: D4
8/17/07:
SQ826 SIN-PVG Availability: D4
8/16/07:
SQ495 DXB-SIN Availability: D4
8/17/07:
SQ806 SIN-PVG Availability: D4
8/17/07:
SQ491 DXB-SIN Availability: D4
8/18/07:
SQ806 SIN-PVG Availability: D4
Just looked it up again at 6:40pm CST on 8/4/07 and reconfirmed again with United that there is no availability on any of these flights in "D".