Sorry about the mangled ascii diagram, it should have looked more like
this:
USER SYSTEM CORP NET
+----------------------------+ |10.0.1.x/24
| virtual IP: | ISP IP: | internet +-------+ |
| 10.0.1.26 | 192.x.y.z |==========| sgw |----| +----+
+----------------------------+ +-------+ |--| |
+----+