After 28 posts, it is looking like the meaning of "grail" varies somewhat from person to person but the implication of a "forever" bike is better agreed upon. I also get the impression that "forever" bikes are the ones more likely to be ridden on a regular basis rather than kept for display or dust-gathering purposes. I was, however, expecting to hear more sentimentality as to why a bike might become "grail" or "forever".
The original question "Grail vs. Forever" arose with me after a recent experience with a lovely Mondia Super - When I was in high school a friend, whose family was in property development and quite wealthy, appeared at my house one day with a sparkling new Mondia that made quite an impression on me. I was very jealous, and I have wanted one of my own ever since. Not long ago, at the Bike Exchange where Cap'n Jonny and I both volunteer (he more than me), a Mondia in my size was donated. Someone else put their name on it and it went up on the third tier of racks where it was hard to get at it. More recently, the original claimant changed his mind and I found the bike available on the "for sale" rack in the front of the shop. "Take it for a ride" they said, "we'll give you a special price", so I did. Well, my reaction to the bike on the road was "Meh", I felt a little crowded thanks to the short top tube and the bike felt twitchy for who knows what reason. I put it back in the rack thinking, beautiful bike but I'd really rather ride my Raleigh. I don't have space in my garage or in my heart for a bike that's just pretty (and/or collectible) but would never get ridden.