If the 50 IS comes - it will be 2.0 or slower. We seem to be getting slower with IS on new glass.
And don't forget that f/2.0 is the fastest that's ever been made with IS, if i'm not mistaken (the EF 200L IS f/2.0). Besides that, there's only a lot of f/2.8 with IS (17-55, 24, 28, 100L, 70-200L, 300L, 400L).
So even a 50mm f/2.0 IS might be pushing it (although 50mm is one of the easiest lenses to make fast, so 50mm f/2.0 IS might not be too hard). Alternatively, might we see a videographers' 50mm f/2.8 IS in the same vein as the 24 & 28 f/2.8 IS we got earlier? (and their pricetags?).
Noone's mentioned that 50mm f/1.3 rumours from last week either. If we get that, then the door's open for a 50/1.2L replacement at f/1.1 or f/1.0... (we wish)