Since I have never used BibDesk in conjunction with TextMate before, we can rule out #3 and I use option #2 only if #1 doesn't work or turn up the option I like.
I'm definitely talking about #1, although I was a bit confused myself, to be honest.
So here are some more details: (1) I have set the variable TM_LATEX_BIB (there are spaces in the path, not sure whether that is important). (2) I type \cite{ and start with the first few letters of a citation (e. g. King for KingSmithVanderbilt:blablabla). The scope is text.tex.latex, meta.citation.latex and constant.other.reference.citation.latex. Sounds alright to me, although you are clearly the expert here. (3) I hit escape and nothing happens, no autocompletion or toggling through my options. Instead, I get OS X' alarm bell. (4) I hit option + escape and everything works just fine. So TextMate definitely finds my bibliography file …
If there is anything I can do for regression, just let me know.
Max
PS I will be on a conference Monday through Thursday, so I won't reply as quickly as I usually do.