Ok, you need to use a combination of "stats items" and "stats cachedump" commands.
Basically, stats items spits out stats for each slab class. Part of that stats is slab class id, which (along with limit number) you need to pass to "stats cachedump".
Of course you need to do it on every server. Alternatively, if you use java whalin client, it kind of supports it already, you just need to call statsItems, parse out slab id and call statsCacheDump (or something like this, check out this link
http://www.google.com/codesearch?hl=en&q=memcached+stats+lang:java+show:cM-7xUSBekQ:c3tBZDsv1vE:cM-7xUSBekQ&sa=N&cd=1&ct=rc&cs_p=http://memcachedtest.googlecode.com/svn&cs_f=src/org/memcached/Test.java).
Here is what I just did on my dev machine and got some keys listed:
marfa:~ boris$ nc localhost 11211
stats items
STAT items:1:number 53
STAT items:1:age 91710
STAT items:2:number 61
STAT items:2:age 91710
STAT items:3:number 6
STAT items:3:age 76285
STAT items:13:number 2
STAT items:13:age 91710
STAT items:14:number 9
STAT items:14:age 76938
STAT items:15:number 8
STAT items:15:age 76302
STAT items:16:number 2
STAT items:16:age 76264
END
stats cachedump 15 100
ITEM UserMemcache_0_19680722 [2036 b; 1216152837 s]
ITEM UserMemcache_0_19680718 [1790 b; 1216152837 s]
ITEM UserMemcache_0_19680717 [1790 b; 1216152837 s]
ITEM UserMemcache_0_19680719 [1790 b; 1216152837 s]
ITEM UserMemcache_0_19680707 [1786 b; 1216152837 s]
ITEM UserMemcache_0_19680708 [2020 b; 1216152837 s]
ITEM UserMemcache_0_19680709 [1833 b; 1216152837 s]
ITEM UserMemcache_0_19680706 [1922 b; 1216152837 s]
END