A very good question. I had to take a guess at whether SQL Server would continue the downward sequence correctly after the IDENTITY_INSERT operation. The correct behaviour is logical, but there have been so many bugs with IDENTITY, it's hard to keep track.

My current favourite example:

https://connect.microsoft.com/SQLServer/feedback/details/492452/generate-identity-value-is-incorrect-after-set-identity-insert-on-on-an-empty-table