AMD isn't leaving CPUs. They simply need to find profit streams of ANY kind.
With the PC market shrinking for the second consecutive calendar year, Intel has essentially pushed AMD nearly to the brink of relevance. AMD's CPU market share went from around 20% to roughly half that in just a year in a price war that they clearly lost. Intel can afford taking a hit on gross margins, while AMD simply cannot.
I think AMD had high hopes that their Athlon MP would give them a promising new life in the more lucrative (low-end) server market but I don't think any big OEMs have signed on yet. Based on that, it's hard to see that their x86-64 CPUs will immediately reap near-term rewards in higher-margin segments.
This isn't so much about CPUs being "fast enough" as some have commented. Yes, the PC market is still very soft, but the problem is that AMD lost a pricing war with Intel, losing half their market share in just one year. If AMD can't even compete on value where they clearly are superior to Intel, then their current CPU business model is basically doomed. I don't think they lost market share due to the GHz wars, but due to lack of industry support from the biggest players, particularly in corporate accounts. For example, Dell's Intel-only success alone has been a continued thorn in AMD's side.