(Thinks: Do I jump into this one?) Oh, what the hey.
I spent way too much time looking into this issue leading up to and after the release of the CPU and GPU die shots. In essence, what some have already said are true:
CPU is 3 Wii CPU cores with alot more cache. There is other easily accessible for those really interested, so I'll just leave it at that. Easier to use than Xbox 360 CPU and Cell but less peak performance. Devs got quite good at working around the PS3/360 CPU weaknesses last gen as well.
RAM is twice as much (accessible) but slower. It has eDRAM as well, kind of like Xenos in the 360 and it seems to have similar bandwidth to the GPU (32 GB/s), but no high speed connection to the ROPs (units used for many things including anti-aliasing, which reduces jaggies).
GPU is a more modern architecture, but not as modern as some people think. It is based on the Radeon HD4000 series, but Nintendo's custom tools make for some differences. There is no indication of significant tweaks to the shader architecture. It has 160 shaders vs Xenos' 240, and with architecture and clockspeed difs that adds up to 176 GFLOPS vs Xenos' 216 GFLOPS (theoretical).
Audio DSP is the same as in Wii at around ~120 Mhz. ARM security processor is also the same as in Wii (ARM926 something).
I think that covers the major points...