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It depends on which APIs you use. If you use any classes or methods that the javadocs mark as restricted, then no, you'll need to sign the application. If not though it should just work.
Part of RIM API classes do require signing to work on real device. Information regarding the signing requirement is included into javadoc documentation. Just inspect your application classes according to the javadoc information. If you have used at least one class that requires signing - the application won't work on real device without signing.
On my experience it is a non-trivial task to make a helpful application based only on classes which do not require signing.
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