Now that you're making the icons for all PAF.exe files look the same--the PortableApps.com favicon with a little USB stick--instead of the app icon, we're gonna wind up with a folder full of sameness icon-wise... just makes it somewhat harder to differentiate between what you're looking for and what you're not. (I'll have to resort to actually reading the names of the files now!) Mostly, though, it gives the impression that you think the app being in PAF format is more important than the app itself.
Not preferred... I'm just sayin'.
Standardized installer icons are pretty much commonplace. If you download Firefox or Thunderbird, for instance -- the non-portable ones -- you get a standard installer icon. If we use the app icons, it can be confusing for users since the taskbar icon will look like the running app's icon in Windows 7 (as well as those of us running Vista with a Windows 7 theme). Even on XP and 2000 it can be confusing.
Most users don't keep installers around (it's not usually necessary) but even if you do, they'll all be organized in alphabetical order by AppId (e.g. FirefoxPortable, OpenOfficePortable, VLCPortable) and then by version number so very easy to find the app you want quickly.
We debated doing a generated icon that combined the app's icon with an installer icon base, but it was tricky code-wise and broke if you were using Windows 2000 or your color depths was below 32-bit. Plus, it would cause issues with certain app logos and trademarks.
Sometimes, the impossible can become possible, if you're awesome!
Does a text file give the impression that it's more important that it's text than it being your shopping list, your to-do list, or your short story? How about a PDF? Installers are not the only thing that use standard installers, and I agree with the reasons for it. Especially as, as JTH says, Microsoft is moving to an icon-based rather than an icon/text based taskbar as seen in Windows 7. Even with text I'd like to differentiate between an installer and a program.
I keep a repository of installers, and I do have a portable folder where I keep stuff downloaded from this and other portable sites, and I prefer uniform icons. If all the icons were the same, it'd be easier (more natural) to focus on the text. Anyway, I just set the folder view to List, so it's easy to read anyway. If you collect programs, you don't keep the folder view on Icons or Thumbnails. It's gotta be List or Details.