You can bash with a shield while retaining its shield bonus to your Armor Class.
Prerequisites: Shield Proficiency.
Benefit: When you perform a shield bash, you may still apply the shield’s shield bonus to your AC.
Normal: Without this feat, a character who performs a shield bash loses the shield’s shield bonus to AC until his or her next turn.
Special: A fighter may select Improved Shield Bash as one of his fighter bonus feats.
Source: Player’s Handbook (version 3.5), page 96.