You can trick the system into allowing enough room for borders by giving the elements negative side margins:
Code:
.centerBoxContents, .specialsListBoxContents, .additionalImages,
.centerBoxContentsSpecials, .centerBoxContentsAlsoPurch, .centerBoxContentsFeatured, .centerBoxContentsNew {
margin-top: 1em;
margin-bottom: 1em;
margin-left: -0.4em;
margin-right: -0.4em;
border-left: 1px dotted #aaaaaa;
border-right: 1px dotted #aaaaaa;
}
Adjust to taste.
Notice that some of the borders are shorter than others - this cannot be overcome without some PHP coding.
You might want to try the Column Divider Pro mod, as it compensates for different height products in a row, at the cost of a bunch of HTML table markup.