Salvete!
I think what Robin is looking for is something like Alliance Plus from Follett, where you can search for a title and pull MARC records from somewhere else. I'm new to Koha (still haven't quite got it running), but I wouldn't think it would offer anything like that. I believe Follett charges for the use of Alliance Plus, I do wonder if you could subscribe to something like Alliance Plus and use it with Koha.
Koha most certainly does have an ISBN search feature where someone can import a MARC record from another catalogue using z39.50. It's accessible from the cataloguing module and doesn't take much to get working; just pick your servers and go. I'm old fashioned and tend to import manually, so if someone else wants to chime in on this, go for it. But, I would guess that if someone was looking to do a title search with z39.50, that probably wouldn't be far off. Koha spanks the crizznap outta Follett, IMHO. Cheers, Brooke