The easy way to get there from the U.S. is usually the nonstop from Houston on Continental. Saturdays and Sundays only. There are flights from Miami, but I've heard lots of horror stories about the connections. TACA from Houston is okay too, but if you're connecting to Houston it's usually cheaper to buy the Continental ticket. You can also do an "open jaw" to San Pedro Sula on Continental on Thursdays I think, meaning you could fly to SPS on a Thursday, take the puddle jumpers to Roatan, fly back direct from Roatan on a Sat or Sun.
The Bay Islands Diver site I linked earlier has some good travel info. There's also a regular on this forum from Houston (I forget his user name) who travels alot to Roatan and knows alot about the flights.