I was wondering if it would be ok to put ribbon around a cake iced in buttercream, or is there some pre-op work that should be done first? Thanks in advance for any help 
I use ribbon on my cakes all the time. It always does fine.
I have put ribbon on buttercream many times with no problems at all. I do no extra prep work, I just make sure that my buttercream has crusted over well, so that I make as little fingerprints as possible while working that close to get it straight! (I also use a small paintbrush to dot the ribbon ends with glucose, it makes a great glue for securing the ribbon to the cake!)
A word of caution though, don't use wire-edge ribbon! I learned this the hard way!!! It has a "memory" from being on the spool and is very hard to straighten to put on the cake!
I've heard some people line the inside of their ribbon before placing it on the cake . . . what on earth would you line it with? Painters tape??? And then, how do you secure the ribbon to keep it in place?
whew .. thanks you guys .. here it is ten o'clock at night and the nasty "what if" thoughts were running through my head like was I supposed to do this or that .... I hate not knowing, good thing I can run back here and ask before the cakes even out of the oven
Just another comment from a ribbon user. There are a couple of photos on my website (see link below). I've not had the grease staining issue that I read about either and as far as I know I don't use any special ribbon - I must have just lucked out when I picked up the cheapest one! 
I actually use the writing tip and run a line of icing along the inside of the ribbon to attach it to the buttercream-iced cake. To get it to lay flat on a round tier, I'll make a small snip in the edge of the ribbon so it can slighlty overlap itself for a better fit, and I try to make all of those in the back of the cake, even tho' you can't really tell the snips are there unless you look for them.
Clear contact paper works. Just cut it to the exact width of the ribbon and stick it to the back side. This prevents any grease from soaking through the ribbon. Corsage pins or "T" pins work well to hold it in place. Just be careful to clean them well before using and make sure to remove the pins before eating 
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