darth_ender said:

Someone once said that the burden of meshing the two trilogies together via fan editing should be placed on the PT, but I've thought that such would be far too difficult in some cases. However, just this morning I actually thought of a simple idea to correct this little mistake. A little voice splicing and cutting to Luke's face could resultin a line such as, "I seem to remember a droid like this one...very interesting." I don't know exactly where to get the "like this one" part, but something along these lines might be doable, simply showing that he vaguely remembers R2, but that 20 years have obscured his memory and he can't be sure.

I kinda feel like he does actually recognize R2. If you look closely at the way he looks at R2 when he comes out of hiding, it always seemed to me that he recognized the droid.

Judging by the fact that he lies about a lot of stuff in Episode IV, he could certainly lie about not knowing the droid.

Also remember that there are many "R2" units. This one happens to be "R2-D2" but the line Leia says about "information is in this R2 unit" implies that there are many R2 units around. R2 is never referred to as "R2-D2" in ANH (at least I don't think) so it's certainly plausible that Obi-Wan just is confused. I mean, he has been just chilling on Tatooine for 20 years, lol.