During all my hollydays I used the n900 "mobile hotspot" as mobile hotspot with ad-hoc mode. And it worked very good.

There is another mobile hotspot application you can buy in Ovi store. I don't know if it supports infrastructure mode.

But in my next hollydays I will use another thing:
Huawei E5. This is very cool and works very fine with n900 and is cheap. It is configured by a web interface
advantages:
- It offers full infrastructure with any simcard. (With security, automatic PIN-dialing, multiple profiles.
- It doesn't eat the battery of my n900
- I don't need a dual sim
- I can use it with abroadband

I like it very much and I am sure that it will work even better than n900 mobile hotspot.