Let's say your driving listening to Spotify and all of a sudden you have a sudden desire to listen to a specific artist. This might not be the perfect solution but a nice option until some real integration is available. Spotify has a not so secret phone number which has an automatically response to a text with a Spotify link. This link can open the request artist right in spotify. Here's what to do:

1. Tell Siri to send a text message to (603) 821-4328 (you can enter this number in your phone as something like "Spotify" to make it easier).
2. Say the artist name and send that text message.
3. Wait a couple seconds and you'll get a text message with a link to open the requested artist in Spotify