I'm looking for a sophisticated and robust java applet (or other) that
will provide some of the functionality of a spreadsheet.

Specifically, must have features include:

1. Direct data entry into cells
2. Copy and paste sets of cells
3. Add/remove columns/rows anywhere
4. Simple formulae such as SUM
5. Simple interaction model with remote servlet
6. Easy to deploy over the web

Nice to have features include:
1. Date validated cells with date picker
2. Validated data entry based on reference data
3. Drop down lists in cells
4. Cell merging
5. Disabled cells
6. Customisable client behaviour

Any advice gratefully received.

Regards
Stevie

Check out the following tutorial:
http://www.fawcette.com/javapro/2003_05/magazine/features/tmanfe/default.aspx