Most of the 10-man instances are generally easy enough to be dropped by a pickup group, in my experience, though probably not for the achievements. A well-geared PuG can roll right through Naxx or VoA without any problems at all. But when you start getting into the achievements (Sarth with drakes) or the higher content (Eye of Eternity is tough unless everyone in there is experienced), then things get a little shady. Which is why lots of PuG leaders will be checking gear and achievements -- they'd rather take along someone who's already done the content than worry about pulling newbies through. Services and reputations can help that a little bit too -- a good friends list can come in very handy for PuGging even hard content.
And I'd say Ulduar is not PuGgable, yet, for a number of reasons. First of all, it's brand new, which means that most pickup folks won't know the fights, and that leads to wipes and only about one or two bosses down. Plus, since it is so new, every raid reset counts, and guilds don't want their raiders using up their resets when there's a guild run later in the week (of course that's not an issue for Naxx, which most guilds have on farm anyway). Though as more people gear up and more guilds make progress in Ulduar, you have to think there'll be PuGs in there as well