For Windows 2008 it may be possible to use MS DSM for all storage systems. It is supported by NetApp, check if it supported by your other vendor.
For 2003 you have to use NetApp DSM indeed. This is always tricky situation. In principle, each DSM should bind to known storage system only, so from this side it is OK. But different vendors may have different requirements regarding driver settings, even support different driver versions. You could try to find a common set of parameters that are supported. The best is to open case both by NetApp and other vendor and ask.
Or you could use third party MPIO drivers like Veritas ☺