There are many clustering options which can give you greater distances now. It depends on what exactly you are looking for. For instance you can do Cluster Continuous Replication (CCR) over many miles, which our company has implemented for our exchange servers. You can do stretch clustering using WANsynch or Standby Continuous Clustering over miles distance as well. Can you be more specific about which clustering method you plan to use?
If you are using synchronized writes between two SAN arrays the maximum recommended distance is ~100 KM. Any further than that and the amount of time that it takes to get between the two data centers will cause the writes to the disks to slow down to much. (This number is based on the speed that data can travel over fiber from site to site.)
If you can provide more info about your data sync plans that will help provide a better answer about the distance the data can travel.