The 501 is based on Atheros' AR5001X "Combo" WLAN chipset. Although Atheros isn't the only vendor who'll be shipping a multi-band chipset , if things go like they did with the 802.11a-only market, I expect that most, if not all, of the cards we'll see over the next few months will be Atheros-based clones of the 501.
Similar to its 802.11a brethren, the 501 requires a CardBus, not a PC Card slot. This is because CardBus is a faster 32 bit bus vs. the slower 16 bit PC Card standard.
Tip: Since CardBus and PC Card products look virtually the same, you'll need to check your laptop's PCMCIA Adapter entry in Windows' Device Manager to see whether your machine supports CardBus. Proxim has a handy CardBus FAQ (PDF format) if you need more info.
The adapter has two LEDs, which blink in unison most of the time, with rates varying according to network activity. You get an alternating blink pattern when the card is searching for an Access Point or other AdHoc client (when in AdHoc mode, of course). The LEDs are at the end of the fairly large antenna housing which will block the CardBus slot above it in a multiple slot laptop. The antenna housing looks like it's about an 1/8th inch higher than the housing on an 11a-only card and has no provision for attaching an external antenna.