Actually, I don't ever remember that working. In fact, I have to switch to S-Video/Composite inputs with the Standard Aspect to watch Standard definition channels in the fullest screen possible. Whereas, for high definition channels I have to switch to Letterbox and Component.
Notice the difference between the two pictures below. Each picture is divided into six screenshots of the Slingplayer Mobile client. The top picture is from the component inputs, and the bottom picture is from the composite. The standard definition channels are all on the left side, and the high definition channel is on the right. The first row shows the standard aspect, the second row letterbox, and the last is pillar.
Screenshot Legend of Component
4:3 Channel - Standard 16:9 Channel - Standard 4:3 Channel - Letterbox 16:9 Channel - Letterbox 4:3 Channel - Pillar 16:9 Channel - Pillar
Screenshot Legend of S-Video/ Composite
Thanks, eferz. I use component input only to cut down on wire clutter (already too many cables on my TV stand.) Maybe I'm insane, but I'd swear the autodetect was working until recently. It's not a terrible inconvenience to change it in the settings, but if I didn't have to, obviously that'd be ideal.