Today I noticed that even my bank is warning people to not do internet banking with Windows XP.
If it is no longer secure enough for online banking it's certainly not secure enough to run a wallet.
(for a node only it would be ok-ish as they have much less confidentiality)
If we consider WinXP that unsafe we should perhaps just drop support for it (which I don`t think is the right thing to do right now), on the other hand I'm not sure a nag-screen or warning message is of any benefit either... people should KNOW by now that support has ended. AFAIK even MS released an update that, once installed, will tell users of WinXP that their OS is unsupported and unsafe in the future.
Don't assume anything. People may not realize just how dangerous it is in the context of bitcoin.
We should indeed drop support for XP, maybe with 0.10. But before that in 0.9.2 I'd like to add some kind of warning.
Closing this, consensus on the mailing list appears to be that this is not needed. We'll see.