Is there any solutions which will allow me to use two sims in my N900? Recently got a Giffgaff sim and I would like to take advantage if the free internet while it lasts but I don't feel like going and buying another phone.
Any help/solutions would be great.

No.

Short of resoldering, rewiring, remanufacturing or reengineering the phone.

Physically there is such an adapter (See http://talk.maemo.org/showthread.php?t=35725)

However as the N900 doesnt to sim services, theres noway of switchign between the SIMs