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SharePoint 2010 doesn't have the PDF Icon out of the box.  It's simple to add it on the web server for on-premises installations. 
However MS has yet to do so for Office365. SO if you're tired of waiting then you can replace the generic icon using javascript like so:
 
{UPDATE} 2011.09.30:  I'VE COMPLETED A SIMPLER AND MORE COMPLETE SOLUTION HERE:
I SUGGEST YOU IMPLEMENT THE ABOVE SOLUTION RATHER THAN THE ONE ON THIS PAGE.
 
Get files on your environment:
 
1) Download these:

2) Upload those files to your Site Assets Library:

https://<yoursite>/siteassets

 

Replace the icons on your Document Library Views:

 

1)  Add Content Editor Web Part at the bottom

2) Edit the CEWP and reference the JavaScript file in the source

'/siteassets/pdficon.js'

3) Save and refresh the page and you'll see the icons like this:
 
PDFIconDocs.PNG

Add the Content Editor Web Part to all your Doc Lib Views.  This could be a lot of work, so it's probably possible to add this script to your master page instead, but I haven't tried that yet.  You can hide the CEWP if you don't want to see it.
 
Explanation:
The List View Web part outputs the following HTML (new lines & indents added for clarity):
 
PDFIconDocSource.png
The JavaScript simply loops through all the <td> elements finding those with the class 'ms-vb-icon'.
Then it gets the title attribute of the <img> tag within the <td> and checks for a pdf file extension
If it's a pdf then the src attribute is changed from the generic icon to the icpdf.gif
 
Here is the JavaScript of the PDFIcon.js  I'm no pro so I won't be impressed if you can do it on fewer lines ;-)

 

I'll try to add this to my PDF Site Template soon.