A while ago people were reporting issues with how Snow Leopard was handling icons, where many files showed only low-resolution icons that resulted in them looking blurred and pixelated. In addition to these, others are reporting problems with Snow Leopard either taking a long time to load icons when scrolling through Finder windows, or not loading icons at all.
Snow Leopard's management of previews in the Finder's seems to have problems with those that are generated by third-party applications such as Graphic Converter. Many applications will generate icon previews, but if some are outputting in either sizes or formats that Snow Leopard is having a hard time with, it may not load them properly. The prevalence of this problem seems to increase as the number of files on the system with custom previews are increased.
When this problem prevents icons from being loaded at all, the system will revert to using only generic application and document icons are displayed for various files. This can happen both directly in Finder windows, but also in other areas where icons are displayed such as the "Open With..." contextual menu.
Fixes for this problem have not been permanent, but should at least allow you to load previews and icons for a while until the problem creeps up again. The following suggestions may help:
Reload the folder views
Using the "View Options" for folders that have this problem, check and uncheck the box labeled "Show icon previews" a couple of times. This should force the Finder to unload and load the icons, and hopefully get them to display properly.
Delete .DS_Store files
Using a utility like OnyX, try removing the system's .DS_Store files, which hold the per-folder settings for whether or not icon previews should be shown. This will result in custom folder views being reverted to defaults, but when used in conjunction with activating the "Show icon previews" option (above), should refresh the settings for icon previews throughout the Finder.
Boot into Safe Mode
A number of small caches and settings are checked when you boot into Safe Mode, which may clear problems with how icons are displayed in the Finder. Reboot the computer and hold the shift key to get into safe mode, from where you might try navigating to the folders with broken icons to see if they're loading properly.
Clear other caches with OnyX
Using OnyX, Snow Leopard Cache Cleaner, or another "maintenance" utility, run a full clean on the system and clear various caches, which upon proper rebuild may not show any errors again.
Remove custom icons and previews
You can try removing various custom icons and previews from documents using a utility like CocoThumbX, which may help reduce the prevalence of this problem.
Overall, while this problem is annoying, it is purely cosmetic and does not affect how affected applications and documents behave. Hopefully Apple will issue an update that addresses this problem, since custom icons and previews greatly appeal to a number of users.