The Mishna in Bikkurim says that a thief cannot bring bikkurim because he does not own the land upon which the bikkurim grew. The Yerushalmi (Bikkurim daf 2 - coming up in a few days in yerushalmi yomi) raises a safeik whether a thief who steals a plant and replants it in his own soil would then be obligated to bring bikkurim. Unlike land, which is not portable and cannot really be stolen from its owner's possession, a thief can be koneh a plant and would be obligated to simply make monetary restitution to the original owner. Since the thief is koneh the plant, why might he not be obligated to bring bikkurim? The gemara explains the safeik is whether we compare bikkurim to korbanos (bikkurim, like korbanos, must be brough to the mikdash), which are invalid even if the thief is koneh the animal. The Ridba"z notes that this gemara serves as a incontrovertable proof to one side in in a major machlokes Rishonim.
The gemara in Bava Kamma 67 quotes Ula as holding that yeiush, the abandonment of hope of recovery by the original owner of stolen goods, is not koneh - a thief who steals a korban may not bring it as an offering even after its owner abandons hope of recovery. Yet, in Bava Kamma 114, Ula himself seems to hold that once yeiush occurs a thief is considered the owner of stolen goods (he must, however, compensate the original owner). Is yeiush koneh or is it not koneh? Tosfos quotes a machlokes: R"I held that yeiush is not koneh, and in the kinyan of the latter case is due to other factors being present (yeiush+shinuy hashem). Rabeinu Tam, however, held that yeiush really is koneh. Nonetheless, even if a thief has a valid kinyan in an animal, Ula in BK 67 is mechadesh that the animal is still disqualified from being offered as a korban because of the principle of mitzvah haba'ah b'aveira, i.e. a mitzvah (like offering a korban) may not be done via improper means (theft).
In our case in Bikkurim, it is clear from the gemara that if bikkurim is treated as a korban, even if the thief has a kinyan in the object, it is disqualified from being used for mitzvah performance, supporting the opinion of Rabeinu Tam.
What exactly is the point of dispute between the R"I and Rabeinu Tam? Stay tuned... (bl"n)