If all else fails try this.
What I've experienced is that this error shows up occasionally after I've copied and pasted existing Ole DB Destination/Source element either from the same Data Flow Task or a Data Flow Task from another package and then start changing the details within.
Even when the Data Type and Length of the columns are identical, this warning still appears on occasion. This is when I delete the copied element(s) and build them from scratch. This has made the warning disappear.
My guess is that the metadata is fouled up during the copy/paste.