Theoretically, no, it's not a problem at all. As long as each device is compliant with the same standard (for example, 802.11b), they should play nicely together.
I have, however, heard of a few quirks in cross-brand networks. My SMC wireless router, for example, had a nasty habit of dumping wireless connections from Macs, which I never quite figured out. By and large, however, such problems are infrequent.
In some cases, there can be benefits to using hardware from the same manufacturer, though. I know that many 802.11b products from D-Link allow you to achieve double the bandwith (or so they claim) of the standard spec, as long as you've got D-Link devices on both ends of the connection.
FYI:
802.11g (which runs at 54 mbps) is backward compatible with 802.11b, though you won't get the additional speed if you mix protocols.