InfoPath Dev

Are you saying you're using the Expression Box to show the value of a certain formula, and you want to format a section based on the same formula?

 There isn't a way to reverence the value of an Expression Box.  You have two options:

1. Incorporate the same formula into your conditional formatting.

2. Use a readonly field that uses that formula as its default value formula, and then reference that field from your formatting conditions.

Expression box control in 2007 probably was substituted by Calculated value control in 2010, which again desn't have underlying data field.

But it is possible to create in a sharepoint list a column of type Calculated (calculation based on other columns) which upon creating Infopath Sharepoint List Form (by clicking "Customize Form" on ribbon) has in Infopath a Calculated control with underlying data field of type CalculatedType which, so, can be referenced. 

Funny but it is impossible to create a data field of such type explictly in Infopath Designer 2010...