On the contrary, while AMD has been leaning on K8 for some time, Intel has actually been juggling several different architectures simultaneously and producing them (and derivatives) on multiple different processes.
They've produced Netburst on the 180nm, 130nm, 90nm, and 65nm processes in single and dual-core variants (90nm and 65nm)
Pentium M was produced on 130 nm (Banias), 90 nm (Dothan), and 65nm (Yonah)
Core 2 has been produced on 65nm and will make the jump to 45 nm (and Core 2 isn't that closely related to Pentium M despite what some say)
Then there's Itanium which has been produced on 180nm, 130nm, 90nm, and 65nm (provided that Montvale is actually released, is it?)
That doesn't include the other chips Intel produces, such as northbridge and southbridge chips for motherboard chipsets, of which they have made many many revisions in the past few years . . . and so far as I know, they've produced those chips on processes as small as 90nm. I don't know if they're producing nb/sb chips on the 65nm process yet.
So, Intel at the very least has been shrinking many different architectures simultaneously, essentially doing two pretty complex things at the same time (if not more than two). Most if not all of their die shrinks have at least coincided with minor architectural changes to each and every different CPU architecture that has experienced a process shrink. I can't think of a single optical die shrink they've performed since the introduction of the Pentium 4, with the possible exception of Presler/Cedar Mill which still had some modifications vs Smithfield/Prescott (l2 cache sizes were different).
I don't think Nvidia and ATI can really use the excuse that their constantly-shifting GPU architectural shifts necessitate slow adoption of process shrinks. They introduce a new GPU once every 12-15 months which, compared to what Intel has done at least, isn't that big of a deal when you take into account that that one GPU design will find its way into nearly every product over 2-3 product refresh cycles.
Both companies (even now that AMD owns ATI) are saddled with R&D budgets much lower than that of Intel's, so they get less work done over time.