Jon Udell talks about ActiveGrid (a clustering LAMP stack).
Linux: OK, Apache: mmmm... OK, Mysql: cough, cough !!, PHP: ewwwwwwwww !!!
Nice marketing pitch. Get your favourite today's acronym, spice it up with the word "grid", mix it all together, and serve cool ! (though these clusters believe me have little to do with actual gridding !!)
The idea is not bad at all, the problem is not in the scalability of the RUNTIME infrastructure, that can be bearable, can take a little work to make everything work, but it's still doable.
The problem with this approach is that PHP (and please excuse me PHP programmers, but it's the cruel truth) is
1) not a tidy language (doesn't encourage modularisation, reutilisation, namespaces, packages, components, nein, nothing, nada)
2) only recently implemented object orientation, expect quirks and pains until a few minor versions more, still inheriting a shitload of awful procedural legacy, often incoherent, inconsistent function naming
3) against urban legends, it's not fast at all, check the benchmarks against Perl, Python and Ruby.
Mysql seems on the way to be fixed, but why not using other readily available, stable and feature complete databases ? Firebird, Postgres and Ingres have had this for years, if not decades now.
The ActiveGrid concept is good, the problem is this infrastructure doesn't live up to the expectations. Nothing near .NET or J2EE.
For an equivalent, more promising stack, I'd look at www.spikesource.com instead.