This happened to my site a year ago or so the same time they had issues with titles showing in lower case. I don't think they were related but i did notice that these pages were getting crawled frequently but they never received the cache link beside them, slowly but surely my serp's started to disappear.
The only way i recovered these Serp's was sending a plee to yahoo, to their due they replied in a very short time with "expect to regain your rankings very soon" that was about 7 months ago.
To my knowledge i wasn't breaking any guidelines and there wasn't any information in the letter i received that mentioned that i had. I didn't change the site design due to Google loving it, so i don't think it was any thing i did.
But with no cache link next to your pages it definitely affects your rankings so similar to G's supplemental index I'd say yes.