I just added a requirement to the BIP 72 (bitcoin: URI payment protocol)
spec:

Wallets must include an Accept HTTP header in HTTP requests:

Accept: application/bitcoin-paymentrequest

... and submitted a pull request so the reference implementation follows
the spec.

Thanks to Stephen/Jeff at BitPay for the suggestion. I'll make a similar
change to BIP 70 and require wallets set Accept:
application/bitcoin-paymentrequestack when sending the Payment and
expecting a PaymentACK message in return.

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Gavin Andresen