Try this, it worked great for me and a few other forum members (thanks to chillipepper for posting this)
so incase anyone else has similar issues....this worked for me;
1) go to settings>applications>manage applications>all>google services framework>clear data
2) go to settings>applications>manage applications>all>market>clear cache and force close
3) go to settings>applications>manage applications>all>market>clear cache and clear data
4) Reboot phone
5) Open and sign out of GTalk, then re-open and sign in.
6) Open Market and accept agreement. Let it load your apps. Close Market.
7) Go to market.android.com and sign in. Go to My Market Account>Settings tab>Edit>Nickname your phone and click Update.
8) Install an app from market.android.com.
except i missed out step 5 as i dont have gtalk...
The actual thread is here:
http://http://androidforums.com/desire-all-things-root/353970-cant-download-app-market-no-backup.html
Hope this sorts it for you. If it does please remember to give thanks to Chillipepper for posting.