I just downloaded poi-3.0.2-FINAL-20080204.jar and made a test with a xls file: getSheetName()of HSSFWorkbook doesn't return correct sheet name, instead, it returns "sheet1", "sheet2"... Another issue is getSheetAt(index) only works for index=0. Trying to debug it, I downloaded source file (poi-3.0.2-beta2), and did the same test, the issues mentioned above doesn't happen. So I guess something wrong happened during project(jar) build. Probably you need a re-build for the jar.
Could you please upload the problem file, so we can test? Also, are you sure you only have one version of the poi jar in your classpath? It sounds like you might have more than one, and that's confusing things.
I found the problem: I modified xls file, but didn't put in right place, so app was still using old file (with sheet name=sheet1). Sorry, my bad. You may close it.