edit: Should point out this was written with v1.9.1.0 in mind. There were various modifications to the RWD theme since 1.9.0.1 so the solution may need adapting.
The accepted solution will end up opening all the tabs in the mobile view. OP mentioned only the description.
To make the first tab start open the answer lies within in skin/frontend/rwd/default/app.js
which handles, amongst various other things, the behaviour of the tabs in the product view.
Investigate the block of code starting at line 872, "UI Pattern - Toggle Content (tabs and accordions in one setup)", specifically lines 938-941:
//Open the first accordion if desired.
if (startOpen) {
dts.eq(0).trigger('click');
}
startopen
determines whether the first tab is triggered. Where is startopen
defined? Line 881:
var startOpen = wrapper.hasClass('open');
The wrapper in this case is the .toggle-content div. It does not, by default, have the class open
- you'll have to add that yourself in your copy of <template-dir>/catalog/product/view.phtml
line 175:
<div class="product-collateral toggle-content tabs">
to...
<div class="product-collateral toggle-content tabs open">
That's all there is to it.