They delivered it in 18 months, working in conditions of absolute secrecy, "under budget and over spec". It was advanced, flexible, skinnable, backwards-compatible and forward-looking. So naturally it died a slow horrible death at the hands of the folks at Nokia HQ.
Contrary to urban myth, Hildon was not simply a UI but a whole platform and used the kernel of the Symbian OS (not Linux), but everything else had to be developed from scratch (application engines, telephoning & messaging stacks, a Java runtime and applications like an office suite).
Conflicting interests (Nokia vs. Ericsson vs. Symbian Ltd vs. Psion vs. vying factions within Nokia) and goals led to increasing difficulties for the Hildon team, who broke away and set up their own company (Mobile Innovation) and successfully pitched their independent vision to Nokia.
But the lack of vision and factionalism within Nokia continued, and Hildon/Series 90, Nokia's chance of a real smartphone OS, was canned after only a few poor hardware releases, in favour of extending the established yet 'challenged' S60 platform.
Mobile Innovation moved on to working on Flash for mobile UIs, again was scuppered by Nokia's bait and switch, and so eventually agreed to be bought up by the makers of Flash, Macromedia.
Whose boss at the time was... well, you'll see when you read the article at The Register.