Up to date as of yesterday:
The Slow Boats and the Speedboats are both running from Luang Prabang to Huayxai and vice versa. The river right now has enough water in it for the boats, but the rapids can prove a little scary at times (out boat literally twisted as it went up the rapids). The slow boat takes two days, stopping in Pak Beng along the way. It will also arrive too late to cross the border in Huayxai, so plan on a night there.
The slow boat dock is 2 kilometres away from Lao Immigration. THere are rickshaws to take you into town, up to you whether or not you take them. Buses to Chieng Mai leave at 10:30 AM and then every few hours.
Speed Boat prices are around 400-500 thousand kip. Slow Boat around 120 thousand for each leg of the journey (LP-Pak Beng; Pak Beng-Huayxai). DO NOT buy your ticket straight to Huay Xai. Half the time you will need to rebuy it in Pak Beng, or so we’re told. Cannot personally verify this.
Regarding the Cambodia-Laos border. Yes, there are now Visas upon Arrival. The price varies, and you need some extra dollars for the immigration people on either side of the border.