K, here's the result of a very short test:
Test was run against a single "a storm spider" (blue con) in crystal caverns.
Total feigns: 110
Failed feigns: 28
= Success ratio: 25.5% (at skill levels 125 to 133 (had a few skill increases), which seems about right)
Successful feigns: 82
Whenever I had a successful feign I let the spider walk back to its spawn spot, where I should remember me sometimes.
Times it remembered me: 0
... ;(
Either Wiz changed this overnight or the chance-to-remember is almost non-existant (like Liam said). The latter seems to be more realistic.
The test was too short (man, that was boring) to be accurate so I'd say that the chance can't be higher than 3 or 4%, maybe even lower.
Now, I have to say that I actually never had a mob remember me when it was solo. If it happened, it was with 2 or more mobs. So I ran another (much shorter) test against two spiders (blizzard and another storm spider).
There's a two spawn at the end of longer tunnel, which provided a pretty safe and accurate testing area, since I could see both spiders and their spawn spots at all times.
I pulled both, feigned and let one spider reach its spawn spot before I standing up. If both spiders walked back at or almost at the same time I let them both reach their spawn spot before trying again to remove any chances of chain aggroing.
I tried this 15 times and not once did the spider at the spawn spot remember me. I know that this isn't anything to count on, but I do know that remembering does happen. I had it happen almost every time with a four spawn, where I was out of chain aggroing range and the 3 mobs at the spawn spots kept reaggroing on me.
More testing needs to be done on this (or maybe Wiz could just post the numbers

).
Anyway, I want some exp now, so I'm off to actually playing the damn game, hehe.