This might be one of those sneaky little euphemisms (like right-sizing for laying off), but as one who loves words, I guess I can make a case for a difference between the two.
As I see it, a skill is the task itself, while a proficiency describes a skill in which you have acquired a certain level of competency/comfort. I hate corporate/career-type jargon and try to see past it whenever possible, but, especially in an orientation phase (new nurse, new unit, new skill), there might be genuine merit in making a distinction between a task and the performer's ability to do it. For example, I can insert an IV but I don't do enough of them to describe myself as proficient.
The potential problem in this area has more to do with the attitude of the institution/unit assessing proficiency. If it's done with sufficient time and grace to let staff feel motivated and safe, proficiency testing can be a boon to all involved. But, if nurses feel like they're under the gun with too little time or training to achieve real competency, then proficiency testing is just one more club to whack people with.
Wouldn't it be interesting if JCAHO realized that patient care quality is very much affected by staff morale and started evaluating THAT!