Oh yes PLEASE! This would be a very good thing to have!

Wow this is freaky. Just yesterday this exact same idea occurred to me as well. Must be some undocumented telepathy feature of Sametime :-).

onClick event of the field:

this.focus();
this.select();

A lot of work?

@Joseph:
your solution does not work in Notes, and it only works on clicking. I need a Notes solution, that also works when e.g. tabbing.
There is a 2 line solution, but I don't want to add it to each field.
BTW this works:

Sub Entering(Source As Field)
Dim ws As New NotesUIWorkspace
Call ws.CurrentDocument.SelectAll
End Sub

The last thing we need is more one-trick-pony options. What if you only want the text selected the first time? What if you only want it selected if the user hasn't changed it? There are too many variables for a checkbox-option to be useful. Consider...

Sub Entering(Source As Field)
Select Case ThisDoc.Description(0)
Case "" , "Enter Description"
Call ThisUIDoc.FieldSetText( "Description" , "Enter Description" )
ThisUIDoc.SelectAll
Case "None"
Call ThisUIDoc.FieldSetText( "Description" , "Description Required" )
ThisUIDoc.SelectAll
End Select

I've yet to come across a requirement to select-all every time a field is entered. I'm not saying it doesn't exist...just not worth complicating the properties dialog with another checkbox. Just code it.

This would be great. Perhaps only when tabbing from field to field (text, numeric).

I have written code to do this so many times that I can do it in my sleep
YES PLEASE let's have it as a default option - for web, client or both