Nope, you don't need a repeater, switch or router - just a cat5/6 cable will do nicely.
I just added a Mac Mini Server with Lion 10.7.3 as a slave to my MacPro running Snow Leopard 10.6.8. In my case, I tested it using a straight Cat5 cable but later found I had a crossover cable made up already so now I am using that. Mac hardware autosenses the polarity anyway so either a straight or a crossover cable should work.
Ports are default configured to DHCP their addresses so you will need to go into SystemPrefs => Network and do some configuration to set up the IP network between the two machines.
One small trap on the MacPro - it has two GigEth interfaces and I believe Ethernet2 is set to innactive by default. Under the list of interfaces is a + and - and then a wheel/cog icon... you need to click that one and choose Make Interface Active (kicking myself for missing this the first time around).
When you get this far, you should be able to look at the Network pages on both machines and see Ethernet1 (or 2 on the MacPro) show up as Connected.
Then you need to Configure IPv4 MANUALLY. Make sure it's a different network to the other interfaces. (assuming you are ok with IP Addressing?) Macs will route automatically so you wont need to set Router(Gateway) and DNS details on these interfaces. For Simplicity, I went with 10.0.0.1 on the MP and 10.0.0.2 on the MBP. Both with 255.255.0.0 subnet mask.
One more tip, click the Advanced button, then choose Ethernet button and set your configure:Manually, Speed 1000bT, Full Duples, MTU 1500. Make sure both Macs have these details set the same.
Hope that gets you working.
Sean