Dates with no day specified automatically become e.g. 01/09/10 - can this be changed?

You can use eomonth if you have the analysis toolpak addin installed - this is easy enough to do but gets impractical if you want to share the spreadsheet with many users.

or

You could reference the cells with a formula like:
=date(year(<cell>),month(<cell>)+1,0)

or

You could add a macro to automatically 'correct' the cells on entry. This is snazzy but almost certainly more hassle than it's worth.

hth