Building Large Scale e-Commerce Systems?

Fross asks: "I am Technical Architect at a reasonably large e-commerce firm, which naturally has a very large and active Website, with tens of thousands of registered users, and millions of page hits a month. The time has come to reimplement the old architecture from the ground up, and I'm looking for other (preferably Open Source, or Open Source-friendly) solutions that will give a faster, more scalable, more reliable, easily maintainable solution than the current implementation (which is running under ASP, on IIS of course). I have a fair amount of experience with Perl, some with PHP, but have never used them for something of this size. Does anyone have experience of deploying something this large with these technologies, and can give some valued information on how these solutions cope?"