Can I use the QIAquick PCR Purification Kit for restriction enzyme cleanup?

Yes. The QIAquick PCR Purification Kit has been used to clean up fragments between 100 bp and 10 kb from a wide range of enzymatic reactions, removing salts, buffers, enzymes, nucleotides, and primers smaller than 40 nucleotides. Reactions that can be cleaned up with the QIAquick PCR Purification Kit include restriction digests, random priming, ligase, kinase, phosphatase, nuclease, nick translation, and cDNA synthesis reactions.
For data and additional information, please see QIAGEN News article Issue No. 5, 1998 "Fast and efficient enzyme removal with QIAquick Spin kits."