Good little thread so I'll resurrect it.

I think in the OT, Yoda and Obi-Wan weren't necessarily concerned about Luke joining up with Palpatine as they were in keeping Luke away from the Dark Side. Rationally, giving in to the Dark Side doesn't mean one is necessarily a Sith (although they're probably bigger experts of the Dark Side than anybody).

And yeah, Obi-Wan and Yoda had their own agenda for Luke. I think the entire point of the prequels was to show the Jedi Order's many and varied moral failings. They were allegedly dedicated to all of these lofty ideals but by the time of the PT, they'd obviously sold themselves to the Republic and jumped when ever the Senate told them to. Qui-Gon was probably the only Jedi of the old ways who followed the will of the Force, even if it ran counter to the will of the Jedi Council, the Republic or anything else, and got labeled as a "rebel" and "maverick" for his actions. Qui-Gon -- independent and attuned to the Force -- is what the Jedi Order should've been; Yoda -- two-faced, dishonest and self-serving -- is what they had become.

I don't think it's a coincidence that the PT Yoda was all about prophecies and the future and all that crap while the OT Yoda lambasted Luke for not paying attention to where he was, what he was doing. The OT Yoda had learned the hard way how bad things can get when a Jedi's focus is too far away from his immediate circumstances.