In my opinion the 'neatest' solution is to use a custom CLR aggregate function. Afterall this is an aggregate function. Happily MS provide a concatenate as an example or custom aggregates (first example):

http://technet.microsoft.com/en-us/library/ms131056.aspx

I appreciate that for many DBAs rolling out CLR code is a bit of a culture change but it only needs to be done once and the performance seems to be fine.