Hi Jim,
On a 6400 running 9.x, CPU0 will be used for host processes and CPU1 will be dedicated to TMM. I'd be concerned if peaks in traffic started regularly using 75% of CPU1. I expect increased latency at 85-95% CPU utilization. If CPU0 gets up to 95-100% CPU utilization for realtime (not "niced") processes, you could see bigd failing to perform monitor checks and pool members flapping. This will have an impact on load balanced traffic as users get bounced around pool members.
At 85 - 95% memory utilization, TMM will start reaping old connections:
sol5670: Overview of adaptive connection reaping
http://support.f5.com/kb/en-us/solutions/public/5000/600/sol5670.html
In v10, if your config is CMP compatible, TMM will run on both CPUs. You'll have more load balancing capacity than on v9 using one CPU.
http://devcentral.f5.com/wiki/default.aspx/iRules/CMPCompatibility.html
Aaron