Here’s the code from the BlackBerry PUSH API about the how to check the Network Coverage on the BlackBerry:

public static boolean hasCoverage() {
       boolean bisCoverage = CoverageInfo.isCoverageSufficient( CoverageInfo.COVERAGE_BIS_B );
       boolean carrierCoverage = false;
       carrierCoverage = CoverageInfo.isCoverageSufficient( CoverageInfo.COVERAGE_DIRECT );
       return bisCoverage || carrierCoverage;
}

[1] http://us.blackberry.com/developers/platform/pushapi.jsp