Apple App Store: Apple set the bar with the App Store and as you browse through their terms and conditions you see that they continue to be the leader with the end user benefiting the most from their policies. You can re-install purchased applications as many times as you want and even install purchased applications on up to five iTunes-authorized devices so your entire household can enjoy applications made with a single purchase.
I always thought there was a refund policy with the App Store (some sites reported as long as 90 days), but reading through the current terms and conditions in paragraph 12 it is clear there is no refund mechanism in place, unless you experience a technical problem downloading the application.
Android Market: Buyers are allowed unlimited re-installs of each application on their device. I looked through all the terms of service agreements and cannot find anything on multiple devices, which isn't that big of a deal at the moment while there are so few Android devices. However, as Android devices do become more readily available this question will have to be answered.
Android Market buyers can get a full refund of their purchase price if they request a refund within 24 hours after purchase. You can get a refund once per application so if you buy it after getting a refund make sure it works how you desire before purchasing a second time.
BlackBerry App World: You can visit the My World screen on your BlackBerry device and re-install purchased applications an unlimited number of times. You can also re-install purchased applications to a new BlackBerry smartphone up to three times a year, which is very reasonable even for those of us who switch devices quite often.
Unfortunately, according to the Terms of Sale all purchases are considered final. If you have a technical issue with the download you can contact customer service for assistance.
Nokia Ovi Store: The Nokia Ovi Store terms state that customers are "limited to one private installation on one Nokia compatible device at any one time." There is no further clarification on this statement so I am not sure if this means you can login to your Nokia Ovi Store account on another device and redownload content to use on that device instead of the original device or if you will have to purchase again. In my limited trials on a Nokia 5800 and E71 it appears that the applications are limited to a single device. The terms do not specify if you can re-install an unlimited number of times like all the other application stores, but with the My Stuff page tied to your account it seems like you are able to do so as needed.
There is no refund policy on applications purchased from the Ovi Store and if the Ovi Store performs as it should then you should not have technical issues with apps. Unfortunately, the Bejeweled game I purchased to test the system does not work on my E71x so I am trying to get a refund now from Nokia.
Which application store is best?: It looks like the App Store wins for multiple device support, which makes sense given that people are likely to have multiple iPhones or iPod Touch devices in a single home. The BlackBerry App World also has a reasonable multi-device policy. The Android Market leads in the refund department with the only official policy that has an easy return policy for applications.
The App Store also wins in sheer number and low pricing of applications at the moment. Nokia has a chance to gain on the others if the recommendation functionality improves as users share their favorite apps.