Title text: I wonder what 2008 meme will go bizarrely mainstream in 2009 like rickrolling did in 2007-2008. I accidentally <noun>? Yo, dawg? Place your bets now!
In the first panel, we see Black Hat telling Cueball that he has hired Rick Astley to show up at a party for a girl. When Cueball assumes Rick Astley is going to perform a live rickroll (sing the song "Never Gonna Give You Up"), Black Hat assures him that Rick is not actually going to sing.
We then see the girl in question (Danish, Black Hat's girlfriend) talking with Ponytail, notice Rick Astley as he enters her party. Like Cueball, Danish expects Rick Astley is going to sing and rickroll her live, and when he does not, his presence causes her to hear the song in her head instead, and she flees the room screaming. Black Hat has essentially rickrolled her in her own mind.
When Ponytail then asks Rick Astley what he did, he merely answers "that, my dear, is how I roll", referencing the "Puts On Sunglasses" meme.
Rickrolling is where one is redirected to a video of Rick Astley singing "Never Gonna Give You Up" via a weblink purporting to be in relation to something else. It was later extended to any situation where the song is used disruptively, such as during a party (as in this situation).
Rickrolling, which first appeared in May 2007 on 4chan, became viral during the later part of 2007 and 2008, and this is referred to in the first part of the title text. The title text then asks what meme will replace Rickrolling in 2009. Although Randall makes reference to the "Xzibit Yo Dawg" and "I accidentally..." photocaptioning memes, memebase entries suggest the "Keep Calm and Carry On" meme may well have become the most popular (and most enduring, as of 2013) meme of 2009.
Although the identity of the girl Black Hat is pranking is not stated by Randall (he refers to her as Girl 1 but he has also never given an official name to "Danish" - that is an explain xkcd name), she is clearly Danish, Black Hat's girlfriend. Danish was introduced in the Journal storyline (377: Journal 2), some time prior to this comic. She may not at first act like it could be expected of Danish - however Black Hat is a master and he would know that "not getting Rick rolled" would freak her out!
- Black Hat: And so I hired Rick Astley to show up at her party.
- Cueball: And rickroll her? Isn't that a little...last year?
- Black Hat: Ah, but he's not going to sing.
- Danish: Hey is that Rick Astley?
- Ponytail: I think it is. He just came in.
- Danish: Oh no. Brace yourself.
- Danish: Wait. He's just standing there. Where's the song?
- Danish: He's staring at me.
- Danish: This is a little creepy.
- Danish: What's going on?
- Danish: That's actually Rick Astley staring at me.
- Danish: What's he doing?
- Danish: Oh God, I keep expecting it, hearing it in my head.
- [Danish pulls at her hair.]
- Danish: Just do it already!
- [Danish runs off stage right.]
- Danish: Aaaaaaaa
- [Danish slams the door leaving Ponytail and Rick Astley]
- Door: Slam
- Ponytail: Mr. Astley?
- Rick Astley: Yes?
- Ponytail: What did you do to her? What was that?
- Rick Astley: That, my dear,
- [Rick Astley puts on sunglasses.]
- Rick Astley: Is how I roll.