I'm thinking of doing something like this for a bridal shower cake I have coming up:

http://cakecentral.com/gallery/2105643

I'm worried, though, that the flowers will come off the buttercream if I only attach it with buttercream - how do you usually attach large fondant / gumpaste flowers to the sides of buttercream cakes?

I usually use piping gel and it does a good job. I've heard other people mention using vanilla extract to moisten the back too but I've never used it. HTH

I've heard of using piping gel - I've used buttercream, too. I'm just worried with the size and weight of the flowers that it wouldn't be enough to hold them to the side of the cake. I had the same dilemma on this cake:

http://cakecentral.com/gallery/1967138

I wanted to put the flower on the side, but I was worried that it would fall off because of the weight.

Maybe you can IM the person who originally did the cake and ask them how they attached their pinwheels to the buttercream?

If you pipe a *open circle* of b'cream it will act as a scution to hold the flowers on.
I have done many, many b'cream cakes w/gp &/or flondant flowers on them.
Roll the gp as thin as possible when making the flower.
As for your 2nd link if you put that flower down at the bottom and stand it up against the cake the board will help hold it along w/the circle of b'cream.

I made one similar to what you are asking about (photo below) and attached it to cream cheese icing and it stayed with no problem, if I remember correctly I believe I placed a piece of waxed paper between the cake and the flower on the top so the gumpaste did not melt. But the side flower was just placed with extra cream cheese icing. HTH

http://cakecentral.com/gallery/2013815

I used piping gel to attach my fondant flower to crusted cold buttercream and it worked great. My flowers were smaller than yours but I also used it on a plaque on the side of a cake and it stuck.

I made a buttercream cake and had to attach fondant butterflies to the sides. I attached spagetti sticks to the backside of the butterflies with melted chocolate and then insterted them into the cake. It worked very well