"Is a withdrawal of Master Limited Partnership units from an IRA or Roth IRA to a regular brokerage account treated as an in kind transfer, or is it treated as a sale by the retirement account and purchase by the regular account?"
Essentially both. Ignoring any transaction fees that might otherwise apply (and ignoring any potential effects on UBTI), an in-kind distribution from an IRA is equivalent to selling the investment within the IRA, distributing the proceeds, then repurchasing the investment outside of the IRA.
"is withdrawal of MLP units from a retirement account to a regular brokerage account with the same owner treated as a sale of the units by the retirement account, or not?"
Your IRA is not you. An IRA is a tax-exempt trust, not a grantor trust. The tax consequences of distributing an investment from an IRA in-kind are the same as selling the investment within the IRA and distributing the cash.
