I have a Canon i550 and I can get this to work with GimpPrint via a Canon S800 driver but only to 600x600. So the iSeries are not fully supported but only partially.
I agree with Martin that Epson are much better supported and with Ian's Hamiltons efforts Epson Multifunction devices are getting support abeit only on the Iyonix at the moment. If I were purchasing a new printer now I would be looking at an Epson.
Also there is a Linux driver that Canon have on their website for the i550, which I have copies of, if someone could utilise this as the core of a driver