I recently decided to build a Tyranid fleet for BFG. One of the great things about a 'Nid fleet is that you can make a whole bunch of ships without buying a single one. You just need some spare 40K parts.
Anyway, I built this beastie out of a Carnifex carapace, 2 barbed stranglers, a set of old scything talons, stealer talons, and 2 plastic spore mines from BFM. Some green stuff was used to bulk it up a bit.
Does anyone have any links to other tyranid conversions for BFG? That would be super useful.