I'd also like to see a tile server that serves up (raw) elevation data rather than imagery. The tricky part is that elevation data has a sufficiently large dynamic range that you can't use most common image formats; but even with a custom tile format this would be useful for e.g. stereoscopic display.

I've been considering hosting one myself (based on USGS data), but only if it seems like there are other people who are interested in such a service.