Per Paizo FAQ: http://paizo.com/paizo/faq/v5748nruor1fm#v5748eaic9qow

Spell-Like Abilities, Casting, and Prerequisites: Does a creature with a spell-like ability count as being able to cast that spell for the purpose of prerequisites or requirements?

Yes.

For example, the Dimensional Agility feat (Ultimate Combat) has "ability to use the abundant step class feature or cast dimension door" as a prerequisite; a barghest has dimension door as a spell-like ability, so the barghest meets the "able to cast dimension door prerequisite for that feat.

Edit 7/12/13: The design team is aware that the above answer means that certain races can gain access to some spellcaster prestige classes earlier than the default minimum (character level 6). Given that prestige classes are usually a sub-optimal character choice (especially for spellcasters), the design team is allowing this FAQ ruling for prestige classes. If there is in-play evidence that this ruling is creating characters that are too powerful, the design team may revisit whether or not to allow spell-like abilities to count for prestige class requirements.

First of all....WTF paizo?!

Second of all WTF PAIZO!!!

Per Paizo FAQ, any character that can use a spell-like ability which is a 1st level Arcane spell autoqualifies for DD at level 6 (with 5 ranks in knowledge (arcana) and draconic as a language).

For example:  Elf with the "Envoy" ART (comprehend languages, Sorc 1), Gnome with Fell Magic or Magical Linguist, Dhampir (Base), Drow with Seducer, Fetchling (base), Ifrit (Base), Sylph (Base), Undine (Base), Duergar (Base), Samsaran (Base), Smurfneblin (Base), Wayang (Base).

All of these examples could say, take 5 levels of Magus, qualify for the class, pick up 7 levels of magus casting after that, and then go into EK, for a build that seems silly beyond measure.  As a house rule, I would suggest that we ignore RAI and RAW in this case, and force a DD to have a spont-arcane CLASS in their build.

WTF PAIZO, indeed!

YUP!! This actually existed in 3.5 too, but it was a little more vague. Its really awesome IMO. Its nice for prc's that have casting prereq's but dont give good casting bonuses (like DD, oldschool Arcane Archer, Arcane Trickster, the like)

How did I not know about this!?!?

Uhhh your not a power gamer?