Update: When I logged in to check today, the problem has been fixed.
I was at an AAJA conference last week. As to be expected, I came home with a pile of business cards. I was trying to be organized about connecting with people quickly, so sat down at the end of the work day to start adding connections to my LinkedIn network.
To my dismay, when I started sending out messages, the system immediately defaulted to the standard, somewhat lame "I'd like to add you to my professional network" message as soon as I selected how I knew the contact - it didn't let me customize my message.
I contacted the folks at LinkedIn, so said they're aware of the bug and working to fix it. In the meantime, if you're sending out messages out there, be warned that what you're stuck with is the standard default message.
I'll update this as soon as they fix it.