It's always been a murky subject, coming up with target arrays that are acceptable to IPSC. For example, the target below does seem to be impractical, however those that have had to find innovative ways to present targets indoors, find that it is a great space saver, and is easy to score.
I'm not arguing for the use of any particular target arrays, simply that it has always been a murky field, where you usually only find out after the fact that an array wasn't acceptable. Well, no longer.
As of July 2013, IPSC has issued a 13 page document entitled "IPSC Target Array Handbook - June 2013", which gives you clear guidelines as to what is an acceptable array. This is certainly a handy addition to any MD or RM's toolbox. For my USPSA readers, please note these arrays do not apply to you.,