From NetApp side there is no problem to increase (or decrease) LUN size up to 10 x original size. Host behaviour depends.
W2k3/w2k8 can easily recognize and use additional space online; you have option to either extend existing partition or create another one. W2k8 can even shrink volume. You do not need SDW for that (but of course need to know how to do it without it)
SDU does not support LUN expansion (did not last time I checked). It supports volume expansion only together with host based volume manager and will create additional LUN and extend volume with it.
Under Solaris there is no official way to extend existing disk; the only way known to me is to use VxVM 4.x or newer that supports it.