Yes it is possible but not feasable. Particularly on a device like the N900. WEP can be cracked from data packet because each IV (initilasation vector) carries a small piece of the password. The algorythms can then work out from enough IVs what the complete password is.

With WPA, you must force a host to deauthenticate a client, forcing them to reconnect. You can then capture what's called a wpa handshake which can be used to find the password. However, you cannot crack wpa, you must bruteforce it. This means that you must compare it against wordlists and if the key isnt in the wordlist, you're not getting the password.

For instance, an 8-digit alphanumeric case-ensitive wpa key would have up to 62771017353866807638357894232076664161023554444640 34512896 different combinations. And this is WITHOUT symbols.

On the same basis, a 64-digit wpa key would have up to 39402006196394479212279040100143613805079739270465 44666794829340424572177149721061141426625488491564 0806627990306816 different combinations.

In short, it's possible but not feasible. Bearing in mind that a device like the N900 could probably only check a couple hundred keys per second. The best you could do is capture the handshake with the N900 then use a desktop to attempt to crack the password.