Not worth replacing screen - you will have same effect after few months of usage. I've yet to seen used N900 without that

The spots are on exact same place when SoC and some other chip is under the screen - probably excessive heat cause it to show "ghosts". running on stock speed, without overclocking, doesn't help.

Anyway, it's really visible only on full white screen, and only if You know what are You looking for - no big deal, really.

Ps.

To be honest, I would probably never buy N900 from any TMO user - people here know too good how much N900 is worth, so prices are always rip-off :P (I bought mine in *perfect* condition, from first "lame" user - no overclocking or even kernel flashing - for ~150 $, *not* pounds ) Also, big chance of being overclocked and whatsnot. Not that I don't overclock myself ;P