myapp.me.uk does not, quite, have the same draw.
To the OP I'd say if you already run it from a .com domain adding .me.uk cant hurt!
If the .me adds to the name (say, for example, find.me.uk) then 100% go for it :D
Trust is a funny thing: if your userbase is hardened hackers like the HN lot then you might end up (at least initially) with a trust issue - but the general public aren't going to care that much.
Though you do raise a good point about people being confused: but if you own .com and .co.uk variants then I doubt it will account for much loss of traffic :D