If you have upgraded your site from a previous version of SharePoint to SharePoint 2010, you have to enable the new user interface (with the Ribbon) on your site before keyboard shortcuts will work as expected in rich text fields.
As shown in this help document for SharePoint 2010, the standard keyboard shortcuts only make the button to make the change active, not actually make the change. For example, pressing CTRL+B would select the "bold" button on the interface, not actually make the text bold. You would then press the Enter key to activate the button. This is also the standard functionality for SharePoint 2007.
- Activate or place focus on the Bold button. CTRL+B
- Activate or place focus on the Copy button. CTRL+C
- Activate or place focus on the Italics button. CTRL+I
- Activate or place focus on the Underline button. CTRL+U
- Activate or place focus on the Paste button. CTRL+V
- Activate or place focus on the Cut button. CTRL+X
By upgrading your UI, you get the standard bold, underline, italics and alignment shortcuts that you get in Word. You also have to be cautious, as pasting text now also pastes the formatting from the source pretty poorly.
- Apply or remove bold formatting from the selected text. CTRL+B
- Apply or remove italic formatting from the selected text. CTRL+I
- Apply or remove the underline from the selected text. CTRL+U