Actually, never anymore. In the first season there was a couple, but nowadays if they ever (and they rarely do) do them anymore, they tie directly into the plot.
IMO, if you ever hear anyone say, "American Dad is just like Family Guy, except not funny.", they obviously haven't tuned in since 2005. The show is so witty and well-done now that it's sometimes almost hard to believe that FG and it share some of the same writers. Family Guy has become a cheap laugh, and the revival was only good enough for 13 episodes (including the "movie", which despite it's shortcomings, was actually really funny), after the summer episodes, the show just turned into utter dreck, as if the writers came back, did all they needed to, and then come to the horrific realization, "They want MORE?!".
I just have to love the ironic roller coaster of the whole AD/FG affair. First American Dad is announced to replace Family Guy, and fans are pleased. Then Family Guy is actually renewed, and fans are ecstatic, but they are surprised when Seth announces that AD is still set to go. So American Dad premieres during Super Bowl (a la the first Family Guy), and everyone hates it, not only for the fact that it wasn't that great, but mostly for the fact that this type of show isn't needed: Family Guy is coming back! So, in May, FG premieres with a kick-ass episode and everyone is assured that American Dad (which gives us another craptacular episode) will be cancelled. While FG's streak continues, AD winds its way down into an eventual hiatus. Yet, to the shock of many, it reappears in September, and something's wrong, because it's not that bad. And on the other end, FG is getting to be pretty tiresome.
Today, many true fans appreciate American Dad more, and Family Guy's wound up as a lifeless bag of jokes (though I do have to say the upcoming Star Wars episode seems like it could be good). I mean, how does a show go from being a replacement, to being a knock-off, to being superior to the show it not only replaced, but was considered a knock-off of?! It's insane, but that's American Dad for ya...[FONT="][/FONT]