We experienced this in April 2007 when the smoke was everywhere. Flying into Chiang Mai was like flying into a war zone with massive plumes of smoke everywhere. It was difficult to breathe and just got worse as we traveled on to Chiang Rai. We woke up at the Anantara the morning after arrival and teh whole place was covered in black ash. We then traveled by boat to Luang Prabang which took 2 days and most of the time we could not see the banks of the river from the.
Things were a little better in LP, less smoke but it still smelt of smoke everywhere. In the end we got a flight out a few days early to Bangkok, where, unbelievably, the air quality was much better!
I have seen no mention of Northern Lao in any reports this year but would be surprised if it is not affected in some way.