I found the same issue, i had converted a vs2003 application to vs2005 and had textboxes which took advantage of the _gotfocus event to popup my own alphanumeric keyboard, this was all ok in vs2003.
After converting, the same gotfocus would popup the keyboard as before but would keep firing it.
After hours and hours of tinterweb meandering and general messing around,
I removed the handles textbox.gotfocus entry and created AddHandlers for them instead
Inside the gotfocus event i added a removehandler ( so it removed the callback )
Did the call to the alpha keyboard
Then reapplied the addhandler.
This seemed to fix it.