Steve Howe can be clearly seen rolling and smoking a joint, and sharing it with Chris, during the making of Going For The One, if any of you have that boot video, check it out. Wakeman never did any drugs but as we all know, drank too much. 20 years clean now though!
Phil Collins, Chester Thompson and Mike Rutherford all smoked plenty of weed, Phil referring to the Lamb tour as his 'grass tour'. Mike did a radio ad where he stated that "drugs were a destructive influence in my life". Phil mentioned a time when he and Chester smoked some Hawaiian pot moments before going on stage and Phil panicked, not remembering the words to the opening song until two seconds before he was due to sing. As far as I know, Tony Banks never touched anything. Hackett, I have no idea. As for Gabriel, he says that he tried hash once and hated it. And according to Phil, he only needed two sips of wine to get giddy, so I think PG is one of those odd types (like Zappa and Fripp) who simply don't need any form of drug.
Alex Lifeson and Neil Peart both smoked loads of pot, and I have a feeling Alex still does. Neil experimented quite a bit throughout his life, but is clean now apart from his scotch (The Macallan - quite expensive).
Tull? No way. Not Ian Anderson anyway.
Sorry if this has all been said, but I could not sift through that whole thread, I just read the first 10 replies or so, and saw a lot of people spewing misinformation.