Joomla! 1.5 RC3 (codename: Takriban) is now available for download and a stable version is "very close".
It’s been 10 weeks since Release Candidate 1 and five weeks since Release Candidate 2. That maybe a good guide to when we can expect the stable version (fingers crossed!).
The SEO is getting better with each version and hopefully the stable version will feature even more improvements.
Overall, if you’re serious about your sites SEO, I’d recommend patience when it comes to upgrading because quite a few important features are still lacking (unique page titles and SEF URL automapping being the two most important) but I’ve no doubt that they’re coming soon, either in the core or via plugins.