I had the same concern because many years ago I had heard and understood the same thing about b/g/n WiFi networks as Babaganosh wrote. Perhaps it was a misunderstanding that has proliferated across the Internet as a strong, yet incorrect, belief.
Thank you, Bob Timmons, for repeating yourself so many times. You've been very consistent with your information.
This concern is particularly troublesome because Apple's website has footnotes that aren't as clear as Bob Timmons has been. For instance, check Apple's product webpages for both the new Airport Extreme (footnote 2) and new Airport Time Capsule (footnote 4). They both contain the following statement that would lend credence to this concern:
"Speed and range will be less if an 802.11a/b/g product joins the network."
The complete footnote, for context; I bolded the prior statement to show its place:
"AirPort Time Capsule is based on an IEEE 802.11ac draft specification. Performance based on comparison with Apple’s 802.11n products. Comparison assumes AirPort Time Capsule network with 802.11ac-enabled computer. Speed and range will be less if an 802.11a/b/g product joins the network. Accessing the wireless network requires a Wi-Fi-enabled device. Actual speed will vary based on range, connection rate, site conditions, size of network, and other factors."
That statement is not unambiguous as yours, Bob Timmons. Its statements like that one in the footnote on the product pages that appear to support the idea that all products must be the same standard, else the whole network will be reduced to and capped at the slowest standard.
Admittedly, the statement does not refer to 802.11n WiFi networks. Are 802.11n devices the exception? Will those not affect the speed and range of the network?
The big question is to what does "speed and range will be less" refer? Does that refer to the whole network, or just the 802.11a/b/g devices. Because 802.11n was not mentioned, that is suggestive that it's NOT only the 802.11a/b/g devices experience slower speeds and shorter ranges. Otherwise, the lack of mentioning 802.11n implies speed and range will NOT be less for 802.11n devices, which suggests 802.11n devices will experience 802.11ac speed and range. That doesn't seem possible. So, "speed and range will be less" must refer to the whole network being capped at max speed and range based on the lowest standard.
I think Apple needs to rephrase that statement for clarity of whatever they were trying to say. I'm not sure what will happen to the quality of the speed and range of the whole network and for each device if devices other than 802.11ac are wirelessly connected.
Until then, it would be much more reassuring if someone can provide a source to an Apple document clearing stating about the 802.11ac supported in the new Airport Extreme and Airport Time Capsule in the way Bob Timmons has stated. The manual and tech specs pages for the new Airport Extreme and Airport Time Capsule didn't seem to have the caveat in any form, and so the footnote on their Overview product pages (but not their Tech Specs page) is reviving this concern at the expense of confusion with their 802.11ac products capabilities.