(1) If the transaction is an asset sale/purchase, then any employment-related agreements with the seller are only assignable from the buyer to the seller if there is an express contractual permission for that.
(2) If the transaction is a stock purchase of the seller, then all assets and liabilities transfer to the buyer, and any employment-related agreements should transfer by operation of law (as a liability).
(3) If the transaction involves a merger of the seller into the buyer or another entity, the outcome becomes more murky, but the analysis is essentially a matter of contract interpretation. The absence of an assignability clause is not fatal, because the definition of employer can be open-ended. State law is critical to consider, but courts are inclined to resolve ambiguities against the employer, as drafter.
See, generally: "Enforceability, By Purchaser Or Successor Of Business, Of Covenant Not To Compete Entered Into By Predecessor And Its Employees" (12 A.L.R. 5th 847, §16).
2012.June.12 General M&A Issues re Benefit Plans and Employment - See Morgan Lewis Article.