Ooo Icelandic volcano thread. Watching with interest.
I feel so much for the residents of Grindavik, though this has been a concern for a couple of years since the recent system became active. The access roads have been at risk previously and because of the gases. That said Grindavik is the greyest most miserable place I've ever been.
Re: blue lagoon. I've been twice. The first time I went it was really nice then it became really commercialised and awful. There are other natural hotwater places in Iceland imho. So if you are going, these are worth considering if the Blue Lagoon remains off limits. (It's been confirmed it's now closed for at least a week). Provided airspace isn't closed due to ash. They are putting in contingency plans for tourism should there be an issue (the main risk to Keflavik is ash. I doubt it's at risk otherwise. There is a smaller airport in Reykjavik itself which probably isn't big enough to run the bigger plans in and out from but they would be able to still get tourists off the Iceland if needed).