Evan, as a Realtor, you have access to past data and trends and should be aware that the Silicon Valley retained a higher percentage of value during the depression than outlying areas. You should also be aware that the percentage of growth during the past 18 months has been less than those in rebounding areas that were more highly depressed. Lastly, I'm confident that you understand that past gains are no guarantee for the future and no one has a crystal ball to see where we are headed.
Overall, the Silicon Valley market has been pounding upward for the past 18 months and finally appears to be slowing to catch it's breath. How will this translate into the coming months? No one knows. You might want to consider reading the following post:
Housing Market Cooling Off: Top 5 Reasons
Because the Silicon Valley is really at the heart of Bay Area values, it is a much safer place to buy so long as the fundamentals are correct.
What you don't specify in your question is whether or not the "buyer" you are referring to is a normal home buyer or an investor: the answer to this question differs greatly based on who is buying the home. If the purchaser is a normal buyer looking to purchase a Silicon Valley home close to good schools and their work AND they plan on living there a ling time, then there is no wrong time to buy. The goal is a roof over their head, not an investment per se. Over time, they will more than likely see increases in the value of their home.
If, however, the buyer is an investor, the answer is different. I personally don't buy investment properties based on speculative return: I buy based on whether of not the property cash flows out of the gate. As prices increase in Silicon Valley, it's becoming very difficult to find properties with a good CAP rate.
Since you are a Realtor, I'd be interested in your take on this.