No metal flashing! Either plastic sheeting or roofing felt should work fine. If you are really worried, you could use a surface membrane like Kerdi from www.schluter.com and use a urethane adhesive to seal the tub junction (they have one called KerdiFix). They have a tile-in expansion joint system that can be used in the corners and at the tub/wall junction, so you don't need any caulk (look at Dilex). I used Kerdi and the Dilex for mine.
Check out www.johnbridge.com for tiling help.