There are several threads on the bitcoin forums proposing bitcoin: URI schemes and a bitcoin-request MIME type. Functionality desired is:

• Click a link on a website
• bitcoin client launches, shows a dialog box with payment details that you then approve to send payment.

Security is obviously a huge issue here; will have to be careful that none of the tricks hackers use to defraud users will work (e.g. clickjacking, HTTP man-in-the-middle rewriting of bitcoin addresses...).