Maybe that's why JKR wanted Kreacher to be left in OOTP, since he'd likely be important in the final book. He only appeared briefly in HBP, but it was important to establish the fact that Harry is Kreacher's master now.
In OOTP, Kreacher was hiding away what Sirius considered 'trinkets'; one of those might be a Horcrux. In fact, wasn't one a crystal bottle of what looked like blood? Crude, but so was the enchantment put on the entrance to the lake where Dumbledore and Harry found the locket. Don't know if Kreacher stole away with it or not.
Can't wait for the last book.
Remember, in HBP, you were required to use blood to get into the chamber, and then you had to drink the potion. Dumbledore also made the point that the boat couldn't detect Harry in the boat because he was not an adult wizard.
Now, what if Regulus brought the blood in the bottle, and took Kreacher with him. He didn't have to weaken himself by cutting, and the boat wouldn't have detected Kreacher, since he is not a wizard. So what if Regulus had Kreacher drink the potion, allowing him to get the locket. That could explain why Kreacher looks so bad and acts crazy - could be a result of the potion.