[toomnyhats]

In customer/main/product.tpl
Find:
<input type="text" size="7" name="price" />

Replace with:

Call for price


If you don't want to hard code it in, you could set up a language variable and just call that. For example: (using whatever css class you desired)

Find:
<input type="text" size="7" name="price" />

Replace With:
<font class="ProductDetailsTitle">{$lng.lbl_call_for_pri ce}</font>