My IRC discussion with Lukas about that issue and the solution:

(18:58:53) yunosh: is it possible that mdb2 and db collide if using 
both in a script, but with different database names?
(18:59:28) lsmith: well .. most db ext reuse connections to the same host
(18:59:42) lsmith: some provide a way to force the creation of a new 
connection
(18:59:54) lsmith: but without it .. you might run into some issues
(19:00:14) lsmith: thats the only point that comes to my head at least
(19:00:27) lsmith: mdb2 does a fair bit of changing in the db sessions
(19:00:32) lsmith: db does much less of that
(19:00:40) lsmith: so if at all .. db could see some "breakage"
(19:02:12) yunosh: well, last time i checked the mysql driver of db 
(which i am using) it performs a select db before each query
(19:02:45) yunosh: and indeed it's a mdb2 query that fails because it 
runs onthe wrong db, not a DB query
(19:06:43) lsmith: ah
(19:06:58) yunosh: got it
(19:07:00) lsmith: yunosh: mdb2 doesnt do the select db
(19:07:10) yunosh: yes, because it tracks the current db internally
(19:07:11) lsmith: so set the new link dsn option
(19:07:30) yunosh: that won't help
(19:07:45) yunosh: oh, it would
(19:07:59) yunosh: but i rather don't want to have two connections open
(19:08:17) yunosh: db selects the db unconditionally
(19:08:30) yunosh: mdb2 only if the internal tracked database name changes
(19:08:36) lsmith: right
(19:08:43) yunosh: and of course it doesn't notice if it's changed outside
(19:08:50) lsmith: you can fix this with a debug handler
(19:09:00) yunosh: like how?
(19:09:15) lsmith: there is a hook that allows you to execute stuff 
before and after a query
(19:09:30) yunosh: ah
(19:10:24) lsmith: http://pooteeweet.org/blog/429
(19:11:39) yunosh: thanks, i guess we'll have to go that route
(19:13:15) lsmith: in the debug handler .. just change the dsn
(19:13:29) lsmith: then it should automatically do the select db if necessary
(19:13:37) lsmith: and skip it if its not needed
(19:14:04) yunosh: well, i could select the db directly in the debug 
handler, no?
(19:14:12) lsmith: you could
(19:14:19) lsmith: but if you execute two mdb2 queries in a row
(19:14:29) lsmith: you would end up with to select db calls (a round 
trip each)
(19:14:45) lsmith: with just updating the dsn you would have to fairly 
cheap php calls and one select db call
(19:15:10) yunosh: well, updating the dsn doesn't help, because it is correct
(19:15:27) yunosh: it just doesn't match the current db link
(19:15:28) lsmith: err .. right

Changes have been made in CVS for this ticket:

http://cvs.horde.org/diff.php/framework/Share/Share/sql.php?r1=1.47&r2=1.48&ty=u

Still cannot reproduce. Is anyone else seeing this?

I am unable to reproduce this.

I've tried enabling the example pref backend source, the favorites, 
the groups, and I also created a new non-shared SQL source.  All 
sources are currently working, and the shared address books are 
showing up as expected.

If using both shared and non-shared sources in sources.php, the "My 
Address Book" screen doesn't show the shared address books anymore.