If you have seen "Playing the Leveling Field" and "Doc Brock," you may have noticed something. In "Playing the Leveling Field," Ash and friends met Fantina. They had a discussion about Gym Leaders. It lead Brock to saying "Amazing. To become a gym leader is not the trainer's goal, but your desire to level up even now? You know, people would do well to start following your example." That got me to thinking that Misty could do that. It would give her reason to travel again. I would also like to add that after hearing Fantina's story, it got Ash excited.
This is what I'm talking about.
Later, in "Doc Brock," Brock decided he'll teach Ash and Dawn how to take care of their Pokemon for when he's not there to help them. It could foreshadow Brock's departure. Funny how the episodes are right by each other. Could that mean that Brock could leave and Misty can take his place?
Another thing I noticed, was the episode where giant Spearow were attacking Ash and Pikachu, thus it may possibly remind him about Misty. (Though she wasn't shown, it's referring to the bike incident.)
Though, not having a cameo yet in the DP series could be because they are waiting to see what Misty's new sprite looks like in Heart Gold and Soul Silver and base the anime version on her new look. Also, seeing Misty again isn't impossible. There have been times where childhood friends/rivals came back. (Kenny was Dawn's childhood friend, Zoey and Candice were childhood friends. Even though Gary wasn't in the Hoenn region at all, he came back a few times.) The lure could have been to introduce new fans to Misty before they get a chance to meet her.
Another thing, is the writers plan on advertising HGSS. (Misty is in HGSS.) There is an upcoming episode where Ash, Brock and Dawn meet Soul from the games. In the original GSC, Misty mentioned something about wanting to travel one day. If she says that again in HGSS, that could mean that Misty's anime counterpart may want to travel as well.