A while back just for the fun of it I wrote a kernel driver whose goal was to extract entropy from the timer interrupt. After all, if anything is predictable, then it'd have to be the timer interrupt.
The point is that while the interrupt is predictable, between the time that the interrupt fires and the driver finally gets run you have cache misses at various levels, PCI bus transfers, DRAM refresh cycles and even just hyperthreading making things very unpredictable. Conclusion: if there's predictability here, I couldn't find it (there's a toolkit for estimating randomness, it concluded that the output was indistinguishable from real random data).
The basic idea was to just use the last few bits of the cycle counter, don't worry about the high order bits. The last bit was enough, but even taking the last four bits didn't show any patterns. It might be worth making such a driver for the purpose of giving otherwise entropy starved machines something to work with. I imagine within VMs the cycle counter becomes even more variable, due to contention with things outside the VM.