I need recommendations for a scalable web host for a high volume WordPress web site. For my purposes, high-volume might be 100K-500K visitors/hour. Might think towards a 1M/hour burst rate as a "high water mark."

I know WP isn't the highest-performing platform out there (ha!), but it's non-negotiable. I can do "the usual optimizations" (e.g. put static files in a CDN, run and follow the advice of performance analyzers like YSlow, etc). But it will still be WordPress, and there will be a dozen or so plugins involved.

So, where to host the site? Most "what's the best WordPress host?" discussions seem to focus on lowest-cost. I need the opposite. What are the high-volume, scalable, or clustered WordPress hosts with which you've had great experiences?