This is really an Ovi Store bug, so if possible, it should be moved forward, but it can be also fixed from the Qt Creator side:

When you package a N900 app, the default filename is something like package_1.0.0.deb. When you upload this to Ovi Store for QA, verification checks fail because Ovi Store thinks that this is not a valid .deb file. If you replace the extra dots with e.g. underscores, this works.

Creator could by default produce correct filenames, but since you can also create valid packages with Scratchbox, it'd be best to also propagate the issue forward.