Science fiction, comic book, and anime conventions often use "con" in their names or as puns. WindyCon, Conclave, MegaCon, Worldcon, CollossalCon, ... Referring to media conventions as "cons" is pretty common in fandom.

Veronica Mars doesn't fit strictly into those categories, but the fanbase seems to have substantial overlap with genre fandom. It's jargon, but not particularly uncommon in its area of relevance.

No, it honestly has a very widespread and global usage. Probably 90% of SF and gaming conventions have a name ending with "Con", or some obvious pun on that. The roots go back at least to the 1939 NYCon I (possibly a retroactive name) or the 1940 ChiCon I.

See for example http://en.wikipedia.org/wiki/List_of_science_fiction_convent....