How do I get ToolValidator to limit the acceptable layer types?

I've been writing ToolValidators for a bunch of tools and this one has me stumped, though it seems it shouldn't. I am working in ArcGIS 10.0 so Composite Types are not allowed.

I have a tool which accepts a Multivalue Layer Parameter. The tool selects Raster Layers and "Point" Feature Layers as valid input from the list, returning a warning string when an invalid layer type is encountered.

Can I get the ToolValidator to accept only raster and point feature layers in 10.0?

Can I at least limit the layers in the multivalue pulldown list to rasters and point layers? I think I can generating the list of valid layers using Describe(), I just haven't found a way to get the ToolValidator to accept only those layers.

Thanks

Paul