Yup, ABC stopped showing it after the cliffhanger at the end of season 2 (Web World War). I think it was two years ago that Reboot got picked up by Cartoon Network and CN aired all 3 seasons and repeated quite a few times. Due to popular demand from fans (like me),
Mainframe decided to continue and wrap up the series with 2 2-hours made-for-tv movies.
However, because of the complex story line development they have, the 2 2-hours movies turned into a full 13 episodes Season 4. ITV in Canada and Cartoon Network in US had already signed on to air the new season possibly in the end of this year or next.
In the mean time, the Season 3 of Reboot had been released on 4 DVDs (4 episodes each). Due to license problem, Season 1 and 2 still can not be released on DVD yet.
Hope that answered your question.
Stay Frosty.