Looks like IBM will be the first company to get a 4GHz CPU into the mainstream. Of course, don't expect this to be in your desktop. At the ISSCC, IBM talked about the next generation of Power CPUs, dubbed Power6. The CPU is designed on the popular "silicon-on-insulator" technology that is capable of reaching 4GHz, 5GHz and even beyond. The particular model they spoke about operated at 5.6GHz.
In one paper, the company described a 5.6-GHz Power6 processor with 64-Kb of Level 1 data cache. The processor is said to have an eight-way, set-associative design with a two-stage pipeline supporting two independent reads or one writes per cycle.
These CPUs are designed soley for servers, and the Power series is known for just that - consuming a lot of power. Performance is likely going to be quite a step up from Power5, and will help IBM keep customers from switching to lower priced but increasingly popular x86 platforms.