Serebii has just confirmed good news. Deoxys is present - and catchable - in Fire Red/Leaf Green.

First you have to get to the Eighth Rainbow Island (that may require something like the Eon Ticket - we don't know yet). On the Eight Island you can catch both Lugia and Ho-oh.

But a Ninth Rainbow Island has been found. (Exactly what you do to get to it isn't clear.) Once there, you simply have to solve an easy puzzle, and Deoxys appears.

She'll be level 30, so catching her should be easy compaired to the 70th-level Lugia and Ho-oh (bring some Dark pokemon).

- Venusaur