The answer is ...YES go ahead and CASE something you've seen in a magazine or on the internet. What's the proper etiquette for CASEing is the bigger question. CASE means "Copy and Share Everything" or "Copy and Steal Everything". My take on this is that where possible always give credit to the original artist. If I've been given a card in a swap I always give credit to the artist. If I've CASED something from the internet (and I always stamp the sample myself!) I give credit to the source of the card and/or artist if I know who it is (sometimes they sign it and sometimes they just have a user name). If I put something on the internet myself I accept that it might be copied for personal use. CASEd artwork should NEVER be submitted for publication.
An example of a CASED card is this one using the Snow Flurries set from the Holiday Mini Catalogue. The original is on the left and was created by my friend Eleonor McGarry. I switched the colours and used the 3/4" circle punch to "pop" up the center snowflake. Same card just different colours and background papers. You could also use this as a template - use designer paper along the side, rip a piece of coordinating cardstock and stick in underneath, pop up your image (switch it out for a flower or other image), add ribbon in the same way and you have a whole new card!
Happy Stamping...and CASEing