[KWH]

First a little history...

*5.1.2 SITUATION B was added to the case book in 2003
5.1.2 SITUATION B was unchanged in the case book in 2004
*5.1.2 SITUATION B was modified in the case book in 2005

New or revised plays in the case book are preceded with an asterick(*) prior to the case play.

Whomever the member responsible for revising *5.1.2 SITUATION B made an error.

In my opinion THE FOLLOWING IS THE CORRECT REVISION:

*5.1.2 SITUATION B: During A1's run for a first down to B's 20 yard line, B12 commits a foul for entering the field during a down. Immediatly following the down, A1 taunts an opponent. RULING: If accepted, the live-ball penalty for B12 entering during a down is enforced from the basic spot and in accordance with the all-but-one principle. After the decision in this foul, A will be penalized 15 yards for unsportsmanlike conduct. (9-5-1a) (9-5-4a) (10-3-2) (10-3-3) (10-4-1) (10-4-4) (10-6)

NOTE:
Before ljudge and others get on here and rip me for quoting the rule or pointing out that the Case Book (Page 71) suggests their might be an exception to this rule let me say this. The CASE Book is a supplement to the rules book. (It says so on page 2)
The Rules we are under obligated to officiate under are found in the RULE BOOK only and no place else.

Historically, a rule change takes a few years to get caught up in the various pulications. eg PSK 2003 (as BOBM metioned above.)

Some people seem to be confused over where the penalty is enforced from if the wording says BASIC SPOT or ALL-BUT-ONE.
For the purpose of of our discussion and sample play; a player, replaced player, or substitute entering during a down (as per 9-6-4a) it makes no difference whether you prefer to say BASIC SPOT enforcment or ALL-BUT-ONE enforcment. If the foul is committed by the offense behind the basic spot the ALL-BUT-ONE shall be used. (SEE Rule 10-6)

Epliogue:
Whomever wrote 5.1.2 SITUATION B AND 9.6 COMMENT in the 2005 CASE BOOK may have misunderstood what was discussed in at the rules commitee meeting in January. Frankly, there was alot of "Frank and Open" discussion regarding this rule change and perhaps more time could have been devoted to this rule. However what was made perfectly clear was this, Under the new rule, if he comes on to the field during live ball play, he participated!
While these case book situations undoubtly can be interpreted to suggest their may be an exception to this rule, if you consult the rule book you will find no such exception.

Best advice for 2005:
1) Keep you flaged tucked neatly into your britches, and pay attention only to fouls which directly affect the play.
2) Make sure a little time is devoted to this situation in your pregame.
3) See number 1 above

I hope this helps.