In Wilcox & Fetzer, Ltd. v. Corbett & Wilcox, read opinion  here , the Chancery Court was presented with the following somewhat unusual question:

 …may Corbett & Wilcox compel Plaintiff Wilcox & Fetzer, Ltd. to arbitrate pursuant to an arbitration clause contained in an agreement to which the former is not a signatory.

Answer: Yes.

See also other recent case summarized on blog  here, that held that non-signatories can compel signatories to an arbitration agreement to arbitrate. One basis for such a finding is equitable estoppel.