The MD has announced that further 737-800 models will be entering the fleet and will be introduced at all bases. They will initially work alongside the -300's and the 757's but will be the long term fleet
Advert for B737-800 captains in flight international says the same. It says the B738 will be working alongside the B733 and B752 at MAN, then progressivly rolled out to other bases.
Im not sure if the B738 is too big for some routes, such as MAN-BUD or MAN-BJV or NCL-KRK, whereas the B733 is more suited (size wise).
Im sure time will tell where they get posted and which routes.
Im guessing the B738 may be initially used on MAN-FAO/ALC/ACE so on, with the B733 doing cities and B752 doing Trunk routes/longer flights (MAN-TLV/PMI/SSH etc)