Renji was indeed a part of the 5th Division at one point.
"2066th term graduate of Shinou-Reijutsuin, the central spiritual arts academy, also known as the Soul Reaper academy. Assigned to fifth company upon graduating, his combat ability was soon recognized and he was transferred to the Eleventh Company. Promoted to Assitant-Captain of the Sixth Company one month ago."
~Renji character profile, vol.9 (Viz edition text).
It was later revealed by Aizen that he had taken not only Renji, but Hinamori and Kira as well, into his Division after their graduation; apparently, his brief meeting with them during that one rigged Academy training mission left on Aizen the impression that the three could be usefully manipulated towards his own ends. Renji, however, proved to be too troublesome and independant-minded for Aizen's tastes (and thus ill-suited for whatever plans he had had for the young Shinigami), and so he was transferred to the 11th Division which not only suited him better but also kept him from influencing the more-useful Hinamori and Kira.
Digital Mage said:
It's just I remember him speaking to Aizen, & he said something like "It's been a while since you were in my division".
Digital Mage, the conversation you are thinking of also occurs in vol. 9, chap. 79, and goes as such:
Aizen: Hello!
Renji: Aizen...
Aizen: Do you...have a moment? *panel, presumably marking a slight passage of time* Hmph...it's been a long time, hasn't it?
Renji: Yes...
Aizen: How many years has it been since I lost you to Kenpachi? You're in the Sixth Company now, aren't you?
Renji: That's right. Um...
The conversation, or at least that portion of it, essentially ends there; they then go on to talk about Rukia and her execution. All dialogue above is written as it appears in the Viz edition.
