Roaming - well it depends on what you mean. Seamless roaming requires either an AP configured as WDS (Wireless Domain Services) and can support up to 60 remote AP's. Cisco also has a dedicated WDS device that takes up a slot in a Catalyst 6500-series switch - the WLSM (Wireless LAN Services Module), this supports up to 300 AP's. You can also cofigure an ISR router as a WDS to support up to 100 AP's.
WDS gives you fast, secure roaming and is generally used for Voice where you need very rapid re-authentication. I don't have any WDS configured and I can happily roam between AP's, although the re-authentication takes a second or so.
Have a read on CCO about WDS if you think this is something you need.
HTH
Andy