Prior to the release of Data ONTAP 7.3.2RC1 a few weeks ago, the new SnapMirror Compression feature was not part of mainstream support. In the past, you would use a WAN accelerator to enable compression of network traffic, minimizing bandwidth consumption. 
Good news is that it's now a standard feature in Data ONTAP 7.3.2, in addition to the bandwidth savings of FlexVol deduplication! See updated documentation on the NOW site:
http://now.netapp.com/NOW/knowledge/docs/ontap/rel732rc1/html/ontap/onlinebk/GUID-B83AC30E-07E2-4E01-9668-5A81C32EF0CF.html
In case you're wondering, native SnapMirror Compression uses the (industry-standard) gzip algorithm, with compression ratios dependent upon type of data.
