For those hoping to test Natty Narwhal 11.04 using Wubi, it will not be fixed in time for Alpha3 (the bug report is available here). The new target for a fix is Beta1.
If you read my previous posts, you will know that you could install Alpha1 with Wubi (with some workarounds), but with Alpha2 a new ubiquity bug prevented this from working. I haven't tested this lately, so I don't know whether ubiquity has been fixed. But it's safe to assume that the only option now for Wubi Natty testing is to install Maverick 10.10 and then upgrade.
It shouldn't really be a surprise. Wubi is intended for the newcomer to Ubuntu so having a working Wubi option early in development is not a top priority. Although I am puzzled what the major difficulty is. Also, the fact that there are so many major changes to grub and ubiquity between Maverick and Natty should be sobering for those that like to upgrade to each new release (these don't seem to be very stable products). When you also factor in the major changes to the front-end (Unity) and it seems to me that the Ubuntu developers are being stretched pretty thin. At the same time there've been some pretty major bugs in Lucid that have gone unpatched for over a year.
Wubi itself is due for an upgrade. With a 2007 version of grub4dos that hangs up on ext4 partitions, and some pretty bad usability bugs (e.g. the python code that chokes on multimedia card readers) and confusing (meaningless) error messages... someone needs to step up.