It usually happens when PayPal needs you to accept the payment in your PayPal account. One typical case is that your customer paid with a currency that is not your default currency for your PayPal account. For example, if your PayPal account is created in France, your default currency will be Euro. If a customer checked out your form and paid with US dollar, you'll see Payment Status at Checkout "pending" for that transaction. What you'll need to do is log in to your PayPal account and accept that payment.
Here is an example screenshot for the PayPal account accepting payments after logging in if you have a pending transaction as I mentioned above. You'll see those pending transaction's Payment status to be "Unclaimed". You can simply click the Accept button to accept the payment. Please note that even if you accept the payment, the payment status in FormsCentral's response table will still show Pending, since so far we are not able to automatically sync the status with PayPal, and that's why we name the column "Status at Checkout", because that is just the payment status when the user checked out the form, not the up-to-date status.
Please let me know if you have more questions.
Senior Computer Scientist
Adobe Systems Incorporated