I returned my Pre to O2 in Dublin at the start of December. I'd had it for about 6 weeks, and it had developed several problems - loose battery, speaker/microphone fail, proximity sensor fail, etc. It hadn't been dropped or abused - just kept carefully in it's sleeve thing and used normally.
O2 insisted that it be sent for repair to a 3rd party, and assured me I'd have it back within 10 working days. They gave me a (horrible) nokia E51 to use in the meantime.
Five weeks+ later, I still haven't got my pre back.
The O2 store say there's nothing they can do, and won't give me an equivalent replacement phone. They said I should contact customer service to enquire about a discount or something. O2 customer service say the E51 is an equivalent phone and there is no grounds for any discount on my bill. (I asked if she'd ever tried writing an email on an E51. . .). Told them I found the whole situation very unsatisfactory, but they didn't seem too bothered. I contacted the 3rd party repair company, and they said that because Pre's are new they don't really know much about them, and they've sent it back to the manufacturer in the UK, and they don't know when they'll get it back.
Argh.
Update: Actually heard from O2 shortly after posting this yesterday, and now have my Pre back. They replaced the speaker and sensor and fixed the other problems. It seems to work fine now. While it was away getting repaired, I got a Touchstone, which is now causing me proplems. . . detailed
here