I'm planning on purchasing a MBA and will likely end up with 128GB as well because I keep most of my iTunes library on external hard drives and will simply sync songs, etc. to my iPhone or iPad. Also, SSD prices will only continue to come down so in a year or two if you find yourself bumping up against the limit of your hard drive, you could purchase and swap in a larger sized drive.
Just a word of caution. Watch the MBA video. Apple take the SSD chips out of the case and put them on the board, to save space. I'm not sure that an SSD upgrade on the MBA is feasible.