"Research In Motion (RIM) must track the use of sensitive application programming interfaces (APIs) in the BlackBerry JDE for security and export control reasons. In the API reference documentation, sensitive classes or methods are indicated by a lock icon or are noted as signed. If you use these controlled classes or methods in your applications, your application must be signed with a key or a signature provided by RIM before you can load the application .cod files onto the BlackBerry smartphone."
For Buying Code signing keys Go Here. You have to spend 20 USD to buy the key.
Some Other Reference link from KB Article.
Also Have a look at this Article. Here signing steps explained clearly.
Once you have the key you can sign as many application as you want.
And just in case if you lost the keys RIM resend you the keys to your mail address by disabling your previous keys. For that you have to send a mail to firstname.lastname@example.org requesting the keys and regarding your issue about key lost. The normally respond within 2-3 business days.
Legally you cannot use the keys more than one PC.
But Practically you can use the keys in any PC. For this:
a. Preserve the following files: i. sigtool.db ii. sigtool.csk iii. sigtool.set b. Copy these files in the JDE bin directory. (ex: C:\Program Files\Research In Motion\BlackBerry JDE 4.3.0\bin)