I love the grass type, and I love Shaymin, so I definitely think he needs a boost. But making him part Ice is not the way to go. The only advantage it offers in neutrality to ice attacks. If they're going to add a second type, it needs to offer more than that. Grass/Ice and Shaymin is still lousy in OU/BL.
EDIT: Just some thoughts on type combos for Shaymin. Grass/Fire has only 3 weaknesses: Poison, Flying and Rock. Not bad. And Grass/Electric is only weak to Fire, Ice, Poison and Bug. And since Poison isn't really a concern at all, Grass/Fire only really has 2 weaknesses to worry about, and Grass/Electric has only 3.