26.8 fits a 28.6 (1 1/8") seat post with a .9mm wall thickness. Not a lot of tube sets available now that use that thick of a seat tube. 27.2 mm seatpost is a more common size, You can use a shim ( available from better LBS's)on the 26.8 post to fit thinner tubes
I neglected to mention that 26.8 can also used with a .8mm wall seat tube, often without reaming.
Yes but, if you use 4130, the tube paramater Velo mentions for 26.8 mm are a comon size, and since SPs are butted on the outside, you can turn down the outside of the tube for a considerable weight savings, or leave it straight for the beef. There isn't a substantian money savings in going the 4130 route since a similar grade of bike tube to aircraft tube is not expensive, but there can be some stock and flexibility issues. Serious builders are doing either one. If you are building road racing bikes you want the real tubes, but for touring, folding, MTB, 29er, you do see some serious folks just using the 4130. Folks are also adding a wide collar at the top of the tube to reduce the distortion that can occur in the cluster area, so overall it's an area where some shop level tube fabrication can be a winner.