Spurred by a comment from Franck Martin on Hurricane Electric's Facebook page
[1], I recalled that someone had in fact done the legwork on which TLDs haven't 
made themselves IPv6-ready yet.  A quick Google ("tld ipv6") led me back to 
Martijn's IPv6 Blog [2] -- unfortunately the post I recalled was 14 months old.

Fortunately, Martijn was kind enough to outline his methodology and link his 
source material, so updating his data set was no trouble at all.  Thanks, 
Martijn!

So, taking from Hurricane Electric's report [3]:

Top Level Domains (TLDs): 294

TLDs without IPv6 nameservers: 51
Percentage of TLDs without IPv6 nameservers: 17.3%

ai ax bb bd bh bo cc ck co cx dj dm ec fk fo gd gf gm gov hm iq jobs kp ky mh 
mil mp mq name ni nu pa pf pk pro pw qa sl sr st td tg tk tl tv uz vu ws 
xn--wgbh1c xn--ygbi2ammx ye

TLDs with nameservers without IPv6 glue in the root zone: 91
Percentage of TLDs that do not have nameservers with IPv6 glue in the root 
zone: 31%

ai aw ax ba bb bd bf bh bj bn bo bs bv cc ck cm co coop cr cx dj dm ec er fk fo 
gd gf gm gov gp gs gu hm hr iq ir jo jobs kp ky mg mh mil mm mp mq mu mv name 
nc ne ni nu om pa pf pg pk pr pro pw qa sb sd sj sl sm sr st sv sy tc td tg tk 
tl to tr tt tv uz vg vi vu ws xn--mgbayh7gpa xn--wgbh1c xn--ygbi2ammx ye zw

Thanks to Mr. Martin for inspiring me to regenerate the list.

--
1. http://www.facebook.com/photo.php?fbid=447358231241&set=a.395837121241.176185.287045761241
2. http://ipv6blog.net/top-level-domains-that-still-do-not-support-ipv6/
3. http://bgp.he.net/ipv6-progress-report.cgi