I hope, you will do good job at the end. IMO, all of GUI archivers look like same. And most of office/home users don't know the correct meaning of archivers. I have lots of funny stories about them. So, best thing for me, keep the archiver as much as user-friendly. Some estimations and some tips in the archiver would be very useful too. So, in the end, you should support both expert and novice users with two different interface. All of these ideas are already planned for BIT. So, you have to make it as soon as possible
For mobile platforms, I highly advice to limit required memory. Most of pocket PC have 64-128 MiB RAM. And most of users don't know the meaning of closing application in a Windows Mobile platform (actually it's some kind of hiding). So, free memory is always low. Also, a light codec (slug, thor, lzss like with filters) can be good too. Because, they are battery powered device and I personally don't want to waste too much time with a compression/decompression process.
Note that, this is already supported by WinRAR
Edit: As a note, most of files which can be opened in pocket PC are small. So, leaving a huge dictionary for a LZ codec is bad IMO. even 4 MB can be high.