I have a Slingbox Pro and am wondering if there is any advantage to upgrading to Slingbox PRO-HD.
Yes, the Slingbox Pro-HD supports high definition streaming whereas the Slingbox Pro does not. The Slingbox Pro accepts high definition signal but it downcoverts for streaming.
Can the iphone 4 access programs on the Slingbox PRO?
Yes, the Slingplayer Mobile for iPhone is offically supported by Sling Media for the Slingbox Pro, Slingbox Solo, and Slingbox Pro-HD. However, in its current state, it doesn't natively support the high resolution. At this time, the highest supported resolution is 320x240 for any Slingplayer Mobile client. So, the picture will be downscaled regardless of which Slingbox you have.