An employer terminated its defined benefit plan and allowed employees a full range of distribution options, including rolling over the money into the employer's 401(k) plan. Is this a related rollover? Is the rollover included in top-heavy testing as part of the 401(k) plan indefinitely? Can the amount be ignored after 5 years since it was part of a distribution to the participant?

You have described a related rollover and it is not counted as a distribution by either plan. It is counted as part of the present value of accrued benefits by the recipient plan for as long as it's there. Although the employee elected to transfer the account to the 401(k) plan, the fact that it is a plan maintained within the controlled group makes it a related rollover.

Help. What about if a group of partners in a law firm leave and start their own company. Would the prior law firm be a related employer or does it lose that relation if the lawyers who leave are no longer partners in the predecessor plan (i.e no common ownership for any period of time). Any help is greatly appreciated.
Thanks

Just to clarify or expound on Jarels comments-

It it irrevlevant to your top-heavy determination whether participants rolled their funds from the DB to the 401(k), took a cash distribution, or rolled them to an IRA.

Distributions Would be added into the top-heavy fraction in both the numerator and denominator for all distributions from the other plan.

Please remember that EGTRRA slightly modified the look back years, etc.