At first I chuckled. Then as I thought about it, I realized there was nothing here but mockery. There are so many assumptions and presuppositions written into those five words that actually communicates the opposite of any belief in Christ's return, or even Christ himself.
I remember Dr. Larson saying once that when he was a kid, growing up in evangelical fundamentalism, which yielded it's corresponding legalism, that if he was in a movie theater when Jesus returned, he wouldn't be taken with the rest of the Christians. This, of course, had nothing to do with salvation, nothing to do with eternal security and nothing to do with living in grace. These kind of Sunday school scare tactics have led to a kind of cynicism that leads to questioning the faith, the validity of salvation and even the eminence of Christ's return.
"Jesus is coming-look busy" implies that even if we believe in Christ's return, he is the mean boss pulling into the parking lot and we have to get back to work, close our solitaire (or worse), and "do Christian stuff." It makes Christ out to be a miser rather than a loving savior who freely gives.
It also implies that Jesus wouldn't know what we were doing before he returns, that he can be fooled or deceived. Underlying this thought too is the idea that "looking busy" is how we earn his approval. It's a works-based theology that takes us back to the middle ages in some ways. I picture the poor uneducated masses, muttering up some prayers paying some indulgences throwing in a little mysticism, and hoping for the best, not really sure if what they're doing will appease a distant Almighty, who hates them because of their sins.
Ultimately, the "look busy" theology identifies itself as a non-belief system. Since we don't know when Jesus is coming (after all, they've been saying that for 2000 years!), and since he evidently hasn't yet, let's go ahead and acknowledge what we're all feeling: we don't really want to look busy. It's far easier to do what we want, rather than to live for they guy who isn't here yet anyway.
As I looked up the words of the bumper sticker I came across an entire line of items with this silly little mantra emblazoned on them. If this is how you want to approach the return of Christ, you can proclaim it in any one of the following ways:
refrigerator magnet 
a tote bag
cooking apron,
mouse pad,
doggy t-shirt, 
shorts,
a badge,
a people t-shirt,
a mug, and my favorite......
a "made in the USA" thong.
