The text format does all sorts of parseFloat. The result of a parseFloat is a number or NaN. The text layer then checks what is the result of these parseFloat calls with things like !== null. If the parseFloat was never reached, the values will be undefined. If the parseFloat was reached, the value will be a number or NaN. Never null. undefined !== null. The real check the text layer should be doing is testing if the value is numeric (see #1441).
The result of all this is that an icon gets created with an offset where x and y properties are NaN. Much later, this value is added to another number and set as a css value with units of "px". Previously, this *appeared* not to be a problem because the Ajax stuff was swallowing errors (don't ask me why, I've spent too much time in the IE debugger already). This weird bug raises it's head when applying the new Request stuff - which doesn't swallow errors.
