Suggestions posted here to take up some other class miss the mark -- this guy wants to know if bardic knowledge is worth a feat, not how he can change classes to get the ability.
With that in mind, I'd say that yes, it is worth a feat. Consider other equivalencies that have appeared in supplemental material. ... The alternate class feature of "bardic knack" in the PH2, for example, can be subbed for b.knowledge, and the knack itself is equivalent to a feat. In Dragon 336, the halfling wizard substitution level gains b.knowledge (as "lore") and loses a wizard feat.
As for the argument that it's unbalanced because the knowledge bonus keeps improving from level (i.e. it's class-based), consider that the "obtain familiar" from Comp.Arc. provides a benefit that does the same -- you get a magical critter that keeps improving, just like the wizard's regular familiar.
Finally, consider the "obscure lore" feat from Comp.Adv., which provides +4 bonus to bardic knowledge checks, which is actually a bigger boost than "skill focus." That might imply (depending on how you look at it) that b.knowledge is somehow less valuable than a regular skill roll and thus needs a larger bonus to compensate. And we know of many feats that allow access to skills themselves. ... Obviously, this point requires sequential inference between several elements.
There are various other feats that provide access to "ability"-like benefits -- the main difference between an ability and a feat is that the former subsumes a feat's prerequisites within the class structure itself. ... So if you don't personally feel that another character should be able to do something specifically bard-like without doing other stuff like the bard class does, set a bardish prereq: max knowledge skill ranks in at least three fields (4 ranks at 1st level, for example) and/or a historic research language like draconic, would seem to justify that the PC in question does, indeed, have a thirst for lore.