But there's an easy sollution for that. I stick a 5cents euro coin in the bases to weigh them down a bit and it works really well. Models don't get knocked over on the table and stay in their place nice and sturdy. I imagine that every currency has a coin with the size of a base so if you're not in the euro zone you might have to have a look wich coin fits. This way you also add a little value to the models. A unit of 10 models costs me €0,50 so that's not much .
But why stop there? I also magnatize all my bases. I started doing this with my fantasy army as they have to be placed on movement trays. so when you magnatize the bases and get a metal sheet and cut that to the size of your unit they will never fall of and you can easily move them araound on the table.
But why do this with 40K models then,.. well you can put your models on any metal tray for easy transport. the models will stick to the tray even if you turn the tray upside down. I have seen DIY cases that fit metal serving trays and have magnetized units on them.Also you can imagine that when you attend a tournament and have to move between tables a lot that it's very easy if you have one or two of these trays and you don't have to worry about people bumping into you when you make your way through the crowded room with your precious painted models.
I then superglue the cut out magnets to the bases.It's better if the magnet is a little bigger then the bases. Because once they are glued properly you can take a knive and cut away the excess.
If you want to see the whole gretchin unit with magnets then click here.