As most of you know, Andy Armstrong, Eric Wilhelm, Leif Eriksen and myself have been working hard to get the new version of Test::Harness out there (Michael Peters, also, but that's for TAP archives for Smolder). You can now download Test::Harness 2.99_01 from the CPAN. Since it's a developer release, you won't get it if you upgrade in CPAN or CPANPLUS. You have to download and install manually. Please don't do this unless you know how to downgrade Test::Harness if things go wrong
Currently, I believe all Test::Harness bugs (out of 46 in the RT queue) are fixed. However, that just means we'll probably find 46 new bugs. For almost everyone, the intent is that the change should be completely transparent. However, there are three significant backwards-compatibility issues:
Test::Harness::Straps is going away by almost unanimous consensus on Perl-QA because it is difficult to work with and relies on knowledge of the Test::Harness internals. You can read chromatic's review of TAP::Parser when he wanted an alternative to Test::Harness::Straps.
The output format changes breaks CPAN::Reporter and causes spurious failure reports (if you're using that module). That's because the old Test::Harness didn't provide rich information and screen-scraping was used to get results. Someone needs to patch CPAN::Reporter to use the new APIs.
You can send bug reports via RT or join the TAP developers mailing list.