Hi Jorgen,
Unfortuantly IEEE802.11F ( the standard for roaming in 802.11 systems) has not yet been ratified and as such roaming on Cisco access points is limited to Cisco client cards only. Currently this is done using the Cisco extensions. Which the Dell cards wont support
This is the same for roaming on other vendors access points also it is limited to only their client cards or client cards from vendors they have shared their roaming system with. Not all AP's on the market support roaming.
Once this standard has been ratified then there will be roaming amongst most vendors also.