Mense,
You brought up a good question. With a block time partner you are only selling time on the jet, not actually part of the title, therefore capital gains is not an issue. Another benefit about block time ( dry lease ) is, it is for a specific period of time. At the end of the lease, usually one year, the parties have the option of renewing it or walking away. As we all know, alot can change with business in a year and if your boss's situation changes he might not need to supplement the jet anymore, or if he "falls out of love" with the block timer, he can find someone else. It also puts you in a good position. If the block timer see he has more use for the plane, he might buy one for himself, and that is great for the industry.
If your boss actually sold a share of the jet, and things didnt work out with the partner, it could get messy when they split up. I've seen this happen with a mooney and a citation I used to fly. One partner wanted to enhance the aircraft and the other liked it the way it was - it created tension.
P3-adub also brings up a good point. The FAA is a good resourse for the wording of the lease. However they are not lawyers so take their recommendations and call your lawyer. The Feds I have dealt with in the past on this subject have really been helpful. ( Imagine THAT )
Another thing Mense, Are you a mamber of NBAA? They have a legal Q&A dept.
Cappy