I haven't actually looked into this question, but I suspect the problem isn't so much that the system isn't responsive when I/O is hogged, but rather that individual programs are not responsive. On my computer, I occasionally get into a congested hard drive issue, and almost all programs I am running are incredibly non-responsive. However, my window manager (awesome), and virtual terminals stay snappy during this period. This is in contrast to when my CPU starts getting thrashed, when even those begin to get unusable.