Tarantula, it doesn't matter if we have a problem with a flat-footed character who is surprised providing flanking. This is the rules forum, not the "how would we like the rules to be" forum.
1) Threatening is not dependent upon the ability to make an AoO. This has been clearly demonstrated in the rules earlier in this thread.
2) There is nothing in the flat-footed rules that states you do not threaten.
Does that make sense? Probably not, but quite a few rules "don't make sense". House rule it if you don't like it.
3) There is nothing in the grapple rules that states you do not threaten.
Summary: If you are flat-footed or grappled you threaten even if you cannot make an AoO because the ability to threaten, by rule, is not dependent upon the ability to make an AoO.