I recently wrote a blog posting outlining the steps you need to take to enable VMware ESX/ESXi's ability to hot add RAM and virtual CPUs to a running virtual machine. Although VMware's compatibility documentation indicates that certain operating systems don't support hot plugging new CPUs, it appears to work for some of them anyway.
If you've successfully hot added RAM or CPU to a running virtual machine, respond to this post with your operating system, edition (standard, enterprise, etc), service pack level/kernel version, and architecture (32/64-bit). If enough people respond, I'll compile all of the results into a usable format and republish it.
A caveat to the ability to hot add RAM and processors. First of all, your operating system has to support the hot add process. Second, you need to be running version 7 of the virtual hardware in vSphere. Version 4 hardware doesn't support hot add.