A while back we reported that Sony Ericsson itself was not being consistent in stating what the internal memory (ROM) was for the Xperia arc. Across a number of official sites, the ROM was reported as anything between 380MB to 1GB. However, Swedroid has confirmed that the Xperia arc comes with 380MB ROM and 512MB RAM. Check out the evidence below along with their unboxing video.
Update: As pointed out in the comments, it looks like the app used to determine the ROM size (Android System Info) does not reveal the total internal memory, just the amount available to the end user. Given that the Xperia X10 has 465MB of free internal memory according to the app, it is feasible that the Xperia arc also comes with a 1GB ROM.