I don't think you have high chances of desoldering base, no matter of what type of soldering iron/torch you use. At most, you will heat up whole ground plane to unhealthy levels.

Prying upwards is IMHO bad idea, too - you may take part of outer ground plane away with you. N900's PCB is 7 layer one - you have nice chances of ripping few ultra-thin tracks, too. But, I have never tried this method (and I'm not planning too), so practice may wary.

Now, the bright part - base of shields isn't titanium it's quite easy to cut off, and I was able to do it with plain kitchen knife, every time I needed.to do such fix. Now, of course, using special carving knife would make life easier, for sure.

Anyway, you just cut it (no ripping, no level'ing) with ease.

/Estel