Unfortunately, http://www.helenos.org/wiki/DiffFromUnix presents it as different enough to make porting software difficult. On the other hand, they say they've got GCC and Python, so it's doable.
I am frankly not very impressed with a quick glance at the networking API. What they show would not be difficult to implement as a library atop sockets, and I'm not convinced their callbacks capture the full flexibility of the old APIs. Meanwhile not providing at least a wrapper to handle the most simple socket scenarios to aid in porting strikes me as a bit stubborn. They determined a priori to hate sockets and that's it.
Some of those actually sound like nice improvements. I think the Unix-style API is in need of a bit of modernization, and it's nice to see someone trying some minor adjustments.
If users need a plain Linux-compatible API, then presumably someone will build a library to handle that. No need to make the raw system call API exactly the same as Linux just for historical compatibility reasons, especially when the opportunity presents itself to clean things up a little.
(I could rant at considerable length about what I would change if I were creating a system-call interface from scratch.)