According to Finnish newspaper Karjalainen (Oct 24. 2011) Nightwish will play a gig "somewhere in the USA" under a disguise name before the Imaginaerum World Tour opening concert at Gibson Amphitheatre in L.A.
Nightwish is known for playing such "hidden" gigs before previous tours in order to test themselves, their crew and all new systems at a live situation before a live audience. Before the DPP tour in 2007 they played three "secret gigs" at small'ish venues in three different European countries.
Whether the "secret gig" will happen in or near L.A. is debatable but likely. Those of you living at the area, better keep your eyes open for what'scoming up at the local smaller venues on the weekdays just before Saturday, Jan 21th. You might get lucky!
Note that this is unconfirmed information aka a rumour. Heck, it's a "secret gig", don't think they'll announce it anywhere with the band's real name, but these rumours have become true before... I attended at one of Nw's disguise gigs back in 2007, bought the ticket on intuition and travelled 4 hrs without any confirmation if I was going to see Nightwish or some other band. Was my lucky day!