Yes, increasing frequency at a certain point would require some increase to timings as the ram modules simply couldn't cope up. The increase in timings has a lower negative effect than increasing the frequency so you'd most likely increase the frequency anyway.
Some may be lowering their ram frequencies just to sync it with the FSB.
Check an old chart here: http://www.tomshardware.com/reviews/sneak-preview,764-7.html
From the image you see that it is faster to have CAS5 800MHz modules than CAS4 533MHz modules. It's harder for manufacturers to lower latency than increasing clock speeds, so the CAS4 533MHz module might end up costlier than the CAS5 800MHz module.