Is anyone using iSCSI over the Internet? How about a WebDAV or SSH/SFTP mapped drive? I would be curious to what other tricks users are doing for offsite backups and how it is performing.

iSCSI over the internet is IMHO a serious mis-design unless this is a dedicated and 1+N redundant leased fiber line e.g. using LACP. If your ISP hiccups your whole system will hang because it will have to wait for the block device to complete an operation. For offsite backups it depends on your data change rate, frequency of backups and then your pipe's speed. For a low cost solution you may try out infinite incrementals to a Samba box and then using Rsync to replicate all new incrementals and collapsed dailies/weeklies/... over to the other side.