There is some confusion about what the Holy Grail really is, but over here there are actually three:
Telecoms: multi-domain roaming.
Voice (or data) calls seamlessly handed-over when the user moves from the reach of one cell (e.g. WiFi) to that of another (e.g. cellular). It is what will enable BT's Bluephone. A sure killer app, that will however drive down revenues of cellular providers, as WiFi calls (essentially VoIP, or wVoIP) are free.
Internet: fully personalized search.
Algorithmic search is getting better. See Yahoo! Mindset, that distinguishes between commercial and research queries. However, any search will normally produce thousands if not hundreds of thousands of results. Users usually concentrate on just the first page. A better understanding of context (see my previous post), user preferences (Google Personalized is an attempt) and the text of a querie will deliver more targeted results.
Media: single-source measurement.
Combination of full media exposure (advertising on radio, TV, print, billboards, internet, theater) and purchasing behavior. Project Apollo (combining VNU's ACNielsen and Nielsen Media Research with Arbitron's Personal People Meter technology, and possibly RFID or smart dust on magazine pages) wants to be the first step.