Actually, they are aware of the problem, unfortunately debugging and reproducing takes a certain amount of time... which is not reduced by shouting at them.
Give them at least some time to resolve the problem.
Could be some change in parsing the mail protocol, but can also be a file system layout conversion of the way they store their mailboxes in Lion as opposed to storing them as they did in Snow Leopard.
Mark, they have called several customers themselves to find the rootcause the problem (yes, that included me.).
Being a software developer myself I can tell you that there is probably a whole process in place they need to go through before they can get the fix out in the field.
Since it is a new OS release they also have to take into account other bugfixes which will all come in one big bundle the next couple of days (Releasing them all seperately will most likely not happen.).
BTW using third party software (open source or not) doesn't make it _not_ their problem, but they are definitely working on it.
Tim