There are actually now two ways to highlight the page you are on. One is with the Page Definition Queries, the other is with some new options in Clip to Shape.
The Page Definition lets you filter features that have attributes that either match, or don�??t match the name of your data driven page index feature. The option to set a Page Definition is a button on the Definition Query Property page, but the button only shows up if 1) you have Data Driven Pages enabled and 2) if the layer is NOT the Index Feature layer.
Using Page Definition Queries, if you want to highlight the page you are on:
1) Copy the index layer to another layer in your TOC and place it above the Index layer in the drawing order.
2) Set the symbol property of the new layer to the color you want to use to highlight the page.
3) In the Definition Query property page of that new layer, select the Page Definition button.
4) Enble the Page Definition Query to "Match" the same attiribute used for your Data Driven Pages.
The help topic for the Page Definition Queries is at:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00sr00000004000000.htm
Clip to shape gives you another way to do this. If you have Data Driven Pages enabled and you go to the Data Frame Properties, under Clip Options there is a new option to "Clip to current data driven page extent." Then there is a button to "Exclude Layers". This lets you pick the layers that you don't want Clipped. You can achive all sorts of interesting results by chosing different layers you want to show in the Index feature area, and features you want to show outside of the Index feature. You might want to play around with this a bit to see what you can acomplish.
The help topic for the new Clip to Shape options is at:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Clipping_the_Data_Frame/00s90000000q00...
I had the same questions and this worked great! Thanks!