I would like to see support for client side SLDs (with WMS-services) in ArcMap. until now (including beta2) only server side SLDs are supported (in layer properties for an WMS (sub-)layer, at the tab "styles".
An option "custom" that would change the URL textbox in this dialog to a read/write state could possible do the trick.
For ArcGIS Server 9.3 some fine samples of the use of a external SLD are shown at:
For on of the samples I made a version with a client side SLD
The original is at:
My adjusted version (with a SLD at my own site is at: