If a portion of your user population has enough trouble with your UI that they are 'fat fingering' their way into trouble, then at some point it is _your_ issue.
But that having been said, a quick glance through the support thread shows things like this: "http://www.google.com/support/forum/p/android/thread?tid=345259e6d424bad3&fid=345259e6d424bad300048dfbff785d0c&hltp=2"
The code reverses the numbers before doing its (loose) compare... so uses the 7 last digits.
Bob - 408-555-1234
Fred - 510-555-1234
And it only uses the first 7 digits, which for both numbers, is "4321555"...
So if you send a message to Fred, and it looks in the cache for the contact, there's a chance it will go to Bob.