What about Thórsmörk and Eyjafjallajökull? 

First a short update about us: 

We are running our tours as scheduled from our farm. We are in no way affected by the eruption. Our horses are outside, and the first foal of the year has been born, and we are expecting three more. They are all out in the fields. For us, the eruption is very far away. 
We did, however, want to keep you updated regarding the Thórsmörk tour, because we are receiving quite a few enquiries regarding this tour in particular.
At the moment, we are quite optimistic that the eruption will calm down soon, and the tours will run as planned. (Please understand: This is not based on facts or anything scientific, just what we think.)
But in case the eruption is still continuing at the time of the tour, this will be our back-up procedure/plan:
If the eruption is still going on in any form (strong, weak, steady), we will not take any risks, and we will redirect the tour to a different area.
If the eruption has stopped and the area is declared to be open by the authorities, we will run the tour as planned.
Please note: Because the eruption can stop just as fast as it started, no firm decision will be made until quite close to the departure date.
What we will offer as an alternative if necessary is a tour with four days' of riding in an area that showcases the diversity of the Icelandic nature. You might want to think in terms of our tour number 11 Krýsuvík or our tour number 12 Thingvellir. We will also offer you a sightseeing trip into Thórsmörk by car, so that you can still experience that area you planned to see. The date/day for such a trip would depend on weather sightings and volcanic activity, of course.