Although it doesn’t have an explicit command for accomplishing focus blending, Photoshop CS4 is very much up to the task using its Auto Align and Auto Blend technologies.
Start by taking a series of photos that shift the focus from the front of your subject to the back. Try to minimize exposure and color/white balance variation. Manual focus, manual exposure and a fixed white balance are going to work best.
Once you have your series of images in a folder, follow these steps:
1 – Choose Files>Scripts>Load Files Into Stack (Click on the “Attempt to Automatically Align Source Images” checkbox.)
2 – Select all the layers in the Layers Panel/Palette (click on the bottom layer, then click on the top layer while holding down the Shift key)
3 – Choose Edit>Auto Blend Layers (Click on the “Stack Layers” option and check “Seamless Tones and Colors”)
Photoshop will create a complex layer mask for each layer, attempting to show only the well focused areas. If you need to adjust for a better result, paint on the layer mask using black to hide and white to show.
A 7MB QuickTime illustrates the steps for this technique.